Merge branch 'more-texture-backends'

Merge branch 'more-texture-backends'

This adds three new texture backends.

- CoglTexture2D: This is a trimmed down version of CoglTexture2DSliced
  which only supports a single texture and only works with the
  GL_TEXTURE_2D target. The code is a lot simpler so it has a less
  overheads than dealing with slices. Cogl will use this wherever
  possible.

- CoglSubTexture: This is used to get a CoglHandle to represent a
  subregion of another texture. The texture can be used as if it was a
  standalone texture but it does not need to copy the resources.

- CoglAtlasTexture: This collects RGB and RGBA textures into a single
  GL texture with the aim of reducing texture state changes and
  increasing batching. The backend will try to manage the atlas and
  may move the textures around to close gaps in the texture. By
  default all textures will be placed in the atlas.