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.
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