cropping: make DisplayWidth/DisplayHeight include cropped area

The primary motivation for this change is to ensure that Matroska files
with `PixelCrop*` elements are displayed consistently, regardless of
whether cropping is applied. This makes such files compatible across
software that either supports or ignores these elements. Without this
change, files suffer from limited compatibility, whereas cropping tags
should enhance the experience rather than be a requirement for proper
image display.

A secondary motivation is that an overwhelming number of files in the
wild do not actually adjust the `DisplayWidth`/`DisplayHeight` elements
as expected by the specification. Most of the time, these values are
left uncropped, with the expectation that cropping will be applied
later. This issue arises partly because MKVToolNix does not
automatically perform this calculation, it only sets the `PixelCrop*`
elements unless explicitly instructed by the user. Observing the
existing files in circulation, it is evident that almost no one manually
adjusts these values.

Lastly, some subtitle formats depend on the video size for positioning
and rendering, making them sensitive to cropping. Currently, most
authoring tools do not take cropping into account. By making the
Matroska specification more flexible and allowing subtitles to be
cropped along with the video, it becomes easier to produce files that
are compatible with various Matroska players.

For a more detailed discussion on this topic, please refer to the links
below:

See: https://gitlab.com/mbunkus/mkvtoolnix/-/issues/2389
See: mpv-player/mpv#13446

Author: kasper93

ebml_matroska.xml

@@ -649,15 +649,15 @@ The format depends on the `ChapProcessCodecID` used; see (#chapprocesscodecid-el
     <extension type="webmproject.org" webm="1"/>
   </element>
   <element name="DisplayWidth" path="\Segment\Tracks\TrackEntry\Video\DisplayWidth" id="0x54B0" type="uinteger" range="not 0" maxOccurs="1">
-    <documentation lang="en" purpose="definition">Width of the video frames to display. Applies to the video frame after cropping (PixelCrop* Elements).</documentation>
-    <implementation_note note_attribute="default">If the DisplayUnit of the same `TrackEntry` is 0, then the default value for `DisplayWidth` is equal to `PixelWidth` - `PixelCropLeft` - `PixelCropRight`; else, there is no default value.</implementation_note>
+    <documentation lang="en" purpose="definition">The width of the video frames to be displayed, including any area cropped according to the PixelCrop* elements.</documentation>
+    <implementation_note note_attribute="default">If the DisplayUnit of the same `TrackEntry` is 0, then the default value for `DisplayWidth` is equal to `PixelWidth`, else there is no default value.</implementation_note>
     <extension type="libmatroska" cppname="VideoDisplayWidth"/>
     <extension type="stream copy" keep="1"/>
     <extension type="webmproject.org" webm="1"/>
   </element>
   <element name="DisplayHeight" path="\Segment\Tracks\TrackEntry\Video\DisplayHeight" id="0x54BA" type="uinteger" range="not 0" maxOccurs="1">
-    <documentation lang="en" purpose="definition">Height of the video frames to display. Applies to the video frame after cropping (PixelCrop* Elements).</documentation>
-    <implementation_note note_attribute="default">If the DisplayUnit of the same `TrackEntry` is 0, then the default value for `DisplayHeight` is equal to `PixelHeight` - `PixelCropTop` - `PixelCropBottom`; else, there is no default value.</implementation_note>
+    <documentation lang="en" purpose="definition">The height of the video frames to be displayed, including any area cropped according to the PixelCrop* elements.</documentation>
+    <implementation_note note_attribute="default">If the DisplayUnit of the same `TrackEntry` is 0, then the default value for `DisplayHeight` is equal to `PixelHeight`, else there is no default value.</implementation_note>
     <extension type="libmatroska" cppname="VideoDisplayHeight"/>
     <extension type="stream copy" keep="1"/>
     <extension type="webmproject.org" webm="1"/>

notes.md

@@ -795,9 +795,22 @@ within the center of a padded 1920x1080 encoded image may set both
 should crop off 240 columns of pixels from the left and right of
 the encoded image to present the image with the pillar-boxes hidden.
 
-Cropping has to be performed before resizing and the display dimensions
-given by `DisplayWidth`, `DisplayHeight`, and
-`DisplayUnit` apply to the already-cropped image.
+Cropping must be performed before resizing to the display dimensions to prevent
+the cropped pixels from affecting the resized image. The numbers given in the
+PixelCrop elements refer to pixels of the stored video before resizing.
+The display dimensions given by `DisplayWidth`, `DisplayHeight`, and `DisplayUnit`
+describe the area occupied by the full image, including the cropped pixels,
+so the actual displayed image may be smaller after cropping is applied.
+
+Whether cropping is applied or not, it should not affect the displayed geometry
+of the image, except for removing the cropped area. After the whole image is
+stretched or compressed in accordance with `DisplayWidth` and `DisplayHeight`
+and the cropped area is removed, the remaining area is not further stretched
+or compressed.
+
+When subtitles are displayed on the video (if relevant to the particular
+subtitle format), they are associated with the full (uncropped) video frame and
+are cropped along with the video.
 
 ## Rotation