This adds a new texture backend which represents a sub texture of a
larger texture. The texture is created with a reference to the full
texture and a set of coordinates describing the region. The backend
simply defers to the full texture for all operations and maps the
coordinates to the other range. You can also use coordinates outside
the range [0,1] to create a repeated version of the full texture.
A new public API function called cogl_texture_new_from_sub_texture is
available to create the sub texture.
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