Don't generate a BufferTextureCopy::rows_per_image if None was specified

[tgecho]

Connections
Fixes #8668

Description
When we generate a fallback value, it causes the gles backend to set UNPACK_IMAGE_HEIGHT to a non-zero value, which causes Firefox to log a warning and not write... sometimes. But None does seem to be the correct value for 1D/2D textures.

Testing
I've tested it manually in context of the app I'm building, but I haven't managed to come up with a self contained test case. There's clearly some other state or something in Firefox that leads to this actually triggering a malfunction.

I'm not quite sure how to add an automated test. Is it practical to write a test where we read back the UNPACK_IMAGE_HEIGHT?

If we're nervous about side effects, I could scope this down to be done just within the gles backend by checking for dst_target == glow::TEXTURE_2D and forcing the UNPACK_IMAGE_HEIGHT value to 0 in https://github.com/gfx-rs/wgpu/blob/trunk/wgpu-hal/src/gles/queue.rs#L754

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.