gsk/gl: pin atlases to single texture library

gsk/gl: pin atlases to single texture library

This removes the sharing of atlases across various texture libraries. Doing
so is necessary so that atlases can have different semantics for how they
allocate within the texture as well as potentially allowing for different
formats of texture data.

For example, in the future we might store non-pixel data in the textures
such as Glyphy or even keep glyphs with color content separate from glyphs
which do not and can use alpha channel only.