texture: Add support for grayscale memory formats

I'm currently getting away with no reallocations from the decoder to a gdk::Texture for the texture data.

The only exception right now is grayscale images. PNG, for example, supports

• Gray8
• Gray8a8
• Gray16
• Gray16a16

Decoders must also deliver them in a non-RGB format since lcms2 denies applying grayscale profiles to RGB data. So I have these internal format anyways.

So it would be pretty convenient if I could just feed those four formats (haven't seen use of any others so far) to gdk::MemroyTexture :)