Fix T84053: Mask overlay in image editor not working

The mask overlay wasn't part of the overlay engine. The reasoning nehind
this was that more editors used the mask overlay and most of them used
old drawing code. This patch adds the mask overlay drawing to the draw
overlay engine. This code path will only be used by the image editor
VSE, Compositor and Movie Clip editor will still use the previous
method.

During this patch some alternatives have been researched:
1. `ED_mask_draw_region`: this would lead to different code paths when
   drawing in the image editor, and some hacks to retrieve the correct
   framebuffer.
2. Add mask drawing to image engine: Would lead to incorrect color
   management when viewing the mask.
3. Add mask drawing to image engine and overlay engine: Would lead to
   duplicated code.
4. Add mask drawing to overlay engine and for combined overlay select
   the correct framebuffer.

Option 4 was chosen as the exception (switching framebuffers) can be
done without hacks. The code stays clean.

2 files changed, 14 insertions, 0 deletions

diff --git a/source/blender/draw/engines/overlay/shaders/edit_uv_image_mask_frag.glsl b/source/blender/draw/engines/overlay/shaders/edit_uv_image_mask_frag.glsl
new file mode 100644
index 00000000000..0b579d6067c
--- /dev/null
+++ b/source/blender/draw/engines/overlay/shaders/edit_uv_image_mask_frag.glsl
@@ -0,0 +1,14 @@
+#pragma BLENDER_REQUIRE(common_colormanagement_lib.glsl)
+
+uniform sampler2D imgTexture;
+uniform vec4 color;
+
+in vec2 uvs;
+out vec4 fragColor;
+
+void main()
+{
+  vec2 uvs_clamped = clamp(uvs, 0.0, 1.0);
+  float mask_value = texture_read_as_linearrgb(imgTexture, true, uvs_clamped).r;
+  fragColor = vec4(color.rgb * mask_value, color.a);
+}
diff --git a/source/blender/draw/engines/overlay/shaders/edit_uv_stencil_image_vert.glsl b/source/blender/draw/engines/overlay/shaders/edit_uv_image_vert.glsl
index d22cc795e66..d22cc795e66 100644
--- a/source/blender/draw/engines/overlay/shaders/edit_uv_stencil_image_vert.glsl
+++ b/source/blender/draw/engines/overlay/shaders/edit_uv_image_vert.glsl