Missing ETC_R11 format

[julhe]

I tried to find the single-channel format from the ETC family, but only found the two-channel variant (SG_PIXELFORMAT_ETC2_RG11). For the BC family, there is BC4, so I guess it would make sense to have ETC2_R11 as well.

[floooh]

Hmm yeah, strange that I overlooked that since the two-channel variants are there...

Technically those should probably be changed to SG_PIXELFORMAT_EAC_* though, see the WebGPU extension (https://www.w3.org/TR/webgpu/#texture-compression-etc2).

[floooh]

Ok fixed in #1044

I actually also found a bug in the WebGPU backend where the RG11 formats were mapped to the wrong WebGPU texture formats.