Enable gamma correction with PyGFX canvas

[gselzer]

PyGFX provides a shader-side gamma within the [renderer](https://github.com/pygfx/pygfx/blob/b865a48ae6268bd1a7c4e6e87eb23d25be2c2d61/pygfx/renderers/wgpu/engine/renderer.py#L110, but ideally we'd be able to apply gamma separately to each dataset being displayed (i.e. to each Image object). Unless I missed something, I don't think this is currently possible.

Naturally, we can also do it on the CPU, as @tlambert03 suggested in #76, but ideally it'd be on the GPU.

@almarklein do you have any insight here?

[almarklein]

This is currently not possible, but adding gamma prop to the gfx.ImageBasicMaterial and VolumeBasicMaterial seems reasonable.