Fix HDR support

[almarklein]

Fixes this wgpu example: https://github.com/pygfx/wgpu-py/blob/main/examples/offscreen_hdr.py

cc @Vipitis

• Fix that the returned bitmap download dict had the format hardcoded to 'rgba-u8' instead of using the actual format.
• Fix the conversion of the buffer bytes to a numpy array, for formats other than u8.
• Performance improvement by caching the parsing of the texture format.
• Add a test to avoid regression.

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The wgpu-py example now produces a nice cube:

[Vipitis]

looks good to me!