Speed up uncompressed DDS reader

[akx]

Follows up on #7589. I noticed this looking at pytest --durations; a fuzzer test that just loads a DDS was surprisingly slow, and turns out doing 1-byte reads is not very efficient...

Locally:

$ pytest-benchmark compare --between=min .benchmarks/Darwin-CPython-3.14-64bit/*.json
Name (time in us)                   0001_orig Min  0002_new Min      ΔMin
-------------------------------------------------------------------------
test_load[uncompressed_rgb.dds]      405,475.7920    1,589.0000    -99.6%
-------------------------------------------------------------------------

or 255.17 times lower minimum, or in average OPS terms, +22351.7% faster. 😂

[radarhere]

An alternative to splitting up PATHS would be to change test_save_jpeg to test_save, and forgo the quality argument.

Is there any reason not to do that?

[radarhere]

As far as uncompressed_rgb.dds is concerned, I think I can do you one better. #7589 added DdsRgbDecoder in order to generalise this decoding path, but before that, this file was read with the raw decoder.

So if apply the following change to main to use the raw decoder for this specific scenario, you will find it faster than the current state of this PR.

diff --git a/src/PIL/DdsImagePlugin.py b/src/PIL/DdsImagePlugin.py
index 40012bc27..a6cec0557 100644
--- a/src/PIL/DdsImagePlugin.py
+++ b/src/PIL/DdsImagePlugin.py
@@ -371,8 +371,11 @@ class DdsImageFile(ImageFile.ImageFile):
                 mask_count = 3
 
             masks = struct.unpack(f"<{mask_count}I", header[84 : 84 + mask_count * 4])
-            self.tile = [ImageFile._Tile("dds_rgb", extents, 0, (bitcount, masks))]
-            return
+            if masks == (0xFF0000, 0x00FF00, 0x0000FF):
+                rawmode = "BGR"
+            else:
+                self.tile = [ImageFile._Tile("dds_rgb", extents, 0, (bitcount, masks))]
+                return
         elif pfflags & DDPF.LUMINANCE:
             if bitcount == 8:
                 self._mode = "L"