Support WGPU_BACKEND and simplify shader util function

[DasLixou]

Two mini patches:

• making the read_draw_tag_from_scene wgsl function more equal to it's cpu equivalent, just from guessing I think this would safe 2 cycles (because 2 add operations are now either 0 or 1) but that's too much of a microoptimization, I just like to think about that.
• Taking WGPU_BACKEND env variable into selection of the backend, which enables me to get rid of the dx12 error spam mentioned in Artifacts on Windows with GPU #449 and also bumps my fps from 80-85 on dx12 to 100-120 on vulkan.

[DJMcNab]

Thanks, both of these look good.

I've also tested the backend selection locally, and was able to get the gles backend working with no apparent compromises