LibWeb: Extract Canvas2DContextBase from CanvasRenderingContext2D

OffscreenCanvasRenderingContext2D duplicates CanvasRenderingContext2D
interface but stubs out every drawing operation, because all the real
recording/present/readback logic was welded to HTMLCanvasElement inside
CanvasRenderingContext2D. Nearly all of that logic is owner-agnostic: it
funnels through the shared canvas command list and drawing state.

Move it into a new abstract Canvas2DContextBase. The element-specific
behavior is reduced to five small hooks (dirty marking after draws, the
Page providing the compositor host, repaint on backing storage creation,
the contextlost/contextrestored event target, and style-resolved
drop-shadow filter colors) that CanvasRenderingContext2D now implements
against HTMLCanvasElement. AbstractCanvasMixin gains set_font and
resolved_letter_spacing as pure virtuals so the base's text code can
reach the CanvasTextDrawingStyles<T> mixin that stays on leaf classes.

This is a pure refactor with no behavior change in preparation for
offscreen canvas support.

Author: kalenikaliaksandr

Libraries/LibWeb/CMakeLists.txt

@@ -489,6 +489,7 @@ set(SOURCES
     HTML/BrowsingContext.cpp
     HTML/BrowsingContextGroup.cpp
     HTML/Canvas/AbstractCanvasMixin.cpp
+    HTML/Canvas/Canvas2DContextBase.cpp
     HTML/Canvas/CanvasDrawImage.cpp
     HTML/Canvas/CanvasFillStrokeStyles.cpp
     HTML/Canvas/CanvasPath.cpp

Libraries/LibWeb/HTML/Canvas/AbstractCanvasMixin.h

@@ -25,6 +25,8 @@ class AbstractCanvasMixin {
     virtual Gfx::Path& mutable_path() = 0;
     virtual Gfx::CanvasCommandList* canvas_command_list() = 0;
     virtual CSS::ComputationContext computation_context_for_drawing_state() const = 0;
+    virtual void set_font(Utf16View font) = 0;
+    virtual float resolved_letter_spacing() const = 0;
     Optional<Color> parse_a_css_color_value(Utf16View value) const;
 };