Clarify how `UNPACK_FLIP_Y` interacts with Exif metadata

The WebGL spec should specify whether `UNPACK_FLIP_Y` flag cancels image-embedded orientation metadata or is applied afterwards.

For comparison, the `ImageBitmap` spec says:
> If the value of the `imageOrientation` member of _options_ is `"flipY"`, output must be flipped vertically, disregarding any image orientation metadata of the source (such as EXIF metadata), if any.

However, the `exif-orientation` test asserts that the embedded orientation is always applied (both for `ImageBitmap` and `HTMLImageElement`) before the image is flipped.

Start by reading the WebGL specification language for UNPACK_FLIP_Y, then compare its intended ordering with the cited ImageBitmap wording and the exif-orientation test. Done means the specification clearly states whether embedded orientation is disregarded or applied before flipping, with the test behavior aligned.

Written by the indexing model from the issue text.