Make handling of non-RGBA sRGB textures more consistent.

Previously, we'd ask the driver to convert these to RGBA, which maybe
isn't ideal, and certainly doesn't work with GLES. Now we send in
the right format for RGB and RGBA, and refuse hardware conversions with
single-channel (which GLES doesn't accept). I don't think this is optimal,
but finding a use-case for sRGB single-channel is a bit tricky anyway,
and the fallback is fast, too.