Auto merge of #24783 - mmatyas:webgl_fns_readpixels, r=nox

Add support for WebGL2 ReadPixels functions

Adds support for the new ReadPixels functions introduced with WebGL2 and the relevant PixelStorei parameters..

Reference: https://www.khronos.org/registry/webgl/specs/latest/2.0/#3.7.10

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This is a work in progress patch, but I think it might be ready for comments. There are a few issues left:

- When the target is the pixel pack buffer, the GL function expects a byte offset as a pointer. In Sparkle the `read_pixels` functions return/work on top of arrays, so for now I've made a [workaround patch](https://github.com/mmatyas/sparkle/commit/45d8bb263d87edcf6c2608fe14d717430025fdc2). I wonder if that's okay or should we do it somehow differently?
- When writing to the pixel pack buffer, padding bytes on the destination are properly ignored. When writing to client buffers, Sparkle `read_pixels` returns a buffer with 1 byte alignment, which I think is fine (less stuff to move between threads), but requires positioning the rows manually (see the bottom of `read_pixels_into` vs. `ReadPixels_`).
- There are some duplicated code between the array buffer and pixel pack buffer variants, eg. the detection of intersection with the framebuffer. This could be refactored, but that results in a function with `Result<Option<Rect<u32>>, WebGLError>`, which I'm not sure is readable enough to help.
- There is a duplication with the WebGL1 code. WebGL2 introduces row length, skip pixels and skip rows as pixel pack parameters which affect the ReadPixels operation. The helper functions could be moved to be usable in WebGL1, but then these new modifiers would also need to be passed as a function parameter, which is somewhat ugly (but would work). What's your opinion about this?

cc @jdm @zakorgy

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