fix: respect source.origin in copyExternalImageToTexture

[Josema]

Problem

The source descriptor already parsed and stored origin, but copyExternalImageToTexture() always uploaded pixels starting at the beginning of the ImageBitmap backing store.

For example, given this 2×2 image:

A B
C D

Copying the right column:

device.queue.copyExternalImageToTexture(
  {
    source: bitmap,
    origin: [1, 0],
  },
  { texture },
  {
    width: 1,
    height: 2,
  },
);

should upload:

B
D

Instead, it uploaded:

A
C

This is especially visible when using texture atlases. Copies intended to generate padding from the last row or column could read pixels from the beginning of the image, producing a solid line along the right or bottom edge.

The existing flipY path had a related issue: it flipped the complete bitmap before applying the copy extent. For a subregion, WebGPU expects only the selected region to be flipped.

Solution

The fix is intentionally limited to the existing copyExternalImageToTexture() implementation:

• Read source.origin, defaulting to [0, 0].
• Validate that origin + copySize fits inside the source bitmap.
• Use the staging path when the origin is non-zero, or when flipY or alpha
  conversion is required.
• Copy only the selected rows and columns into the staging buffer.
• Reverse the selected rows while copying when flipY is enabled.
• Apply alpha conversion to the staged subregion when required.
• Upload the staging buffer with row layout matching the subregion.

The existing direct upload path remains unchanged for zero-origin copies that
do not require flipY or alpha conversion.

Validation

The regression suite covers:

• A non-zero horizontal origin.
• A non-zero vertical origin.
• source.origin combined with alpha conversion.
• source.origin combined with flipY.
• Horizontal and vertical out-of-bounds source regions.
• The existing zero-origin path.

Verified with:

yarn workspace react-native-webgpu test:ref \
  --runTestsByPath src/__tests__/ImageBitmapOrigin.spec.ts

yarn workspace react-native-webgpu test:node \
  --runTestsByPath src/__tests__/ImageBitmapOrigin.spec.ts

yarn workspace react-native-webgpu test \
  --runTestsByPath src/__tests__/ImageBitmapOrigin.spec.ts

Results:

• Chrome reference client: 7/7 passing.
• dawn.node: 7/7 passing.
• iOS Simulator (iPhone 11, iOS 26.2): 7/7 passing.
• TypeScript and ESLint checks passing.
• Native C++ source compiled successfully.

[wcandillon]

@Josema I see it marked as draft, let me know once it is ready for review 👍

[Josema]

Ready. Again, this is a fix from Codex that works for me. Feel free to close the PR if you don't think the fix is good enough.