ETC2 pixel format cleanup

[floooh]

• rename SG_PIXELFORMAT_ETC2_RG11 to SG_PIXELFORMAT_EAC_RG11
• rename SG_PIXELFORMAT_ETC2_RG11SN to SG_PIXELFORMAT_EAC_RG11SN
• add SG_PIXELFORMAT_EAC_R11 and SG_PIXELFORMAT_EAC_R11SN
• fixed a related pixel format mismatch in the WebGPU backend (RG11 sokol-gfx formats were mapped to R11 WebGPU formats)

Also see https://github.com/floooh/sokol/issues/1041