They are not supported in OpenGL Core profile, and we don't have too
many grayscale textures to make this worth the cost.

The main problem with them is that they are incompatible with sRGB
internal format that can let texture sampling automatically yield
linear colors.

Sampling from such textures will automatically convert sRGB-encoded data
to linear color values.
For now all modules will still treat textures the old way.

All the rendering is now supposed to happen in linear-sRGB color space,
and the OETF is applied at the end of rendering, followed by dithering.

…light rendering