The webGL backend uses textures to upload quads to the GPU, since it
doesn't have storage buffers. Each quad is stored in 10 texels, and we
previously fetched each texel 4 times. Although texel fetches are
cached, the extra bookkeeping optimized badly, and lead to bad
performance.

This change replaces the `read_word` calls with `fetch_texel_instance`,
and manually unrolls this loop. This gave the following frame time
improvements scrolling through delta web:

- desktop, 9950x3d iGPU, 1440p, firefox:  ~3.3x faster
- mobile, Pixel 10 Pro, firefox: ~23x faster

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Release Notes:

- [GPUI] Improved performance of webGL quad upload, around a ~3x frame
time reduction in typical cases