Auto merge of #23226 - mmatyas:webgl_compressed_textures, r=jdm

Add initial support for WebGL compressed textures

This patch is an initial implementation of WebGL compressed texture support, it contains

- functions for registering and querying compressed texture extensions
- initial implementation of `CompressedTexImage2D` and `CompressedTexSubImage2D` and their parameter validation
- implementation of S3TC (DXT1, DXT3, DXT5) and ETC1 extensions as examples

What's still missing:

- some of the parameter validation steps are missing
- the pixel comparison tests fail for more complex cases (I'm probably missing something trivial at the GL calls)

Related: #10209 and #20594

cc @jdm @zakorgy

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] Related issues: #10209, #20594
- [x] There are tests for these changes

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components/canvas/webgl_thread.rs

@@ -1168,6 +1168,43 @@ impl WebGLImpl {
                     &pixels,
                 );
             },
+            WebGLCommand::CompressedTexImage2D {
+                target,
+                level,
+                internal_format,
+                size,
+                ref data,
+            } => {
+                ctx.gl().compressed_tex_image_2d(
+                    target,
+                    level as i32,
+                    internal_format,
+                    size.width as i32,
+                    size.height as i32,
+                    0,
+                    &*data,
+                );
+            },
+            WebGLCommand::CompressedTexSubImage2D {
+                target,
+                level,
+                xoffset,
+                yoffset,
+                size,
+                format,
+                ref data,
+            } => {
+                ctx.gl().compressed_tex_sub_image_2d(
+                    target,
+                    level as i32,
+                    xoffset as i32,
+                    yoffset as i32,
+                    size.width as i32,
+                    size.height as i32,
+                    format,
+                    &*data,
+                );
+            },
             WebGLCommand::DrawingBufferWidth(ref sender) => sender
                 .send(ctx.borrow_draw_buffer().unwrap().size().width)
                 .unwrap(),