Instead of directly banging GL to migrate textures the atlas now uses
the CoglFramebuffer API. It will use one of four approaches; it can
set up two FBOs and use _cogl_blit_framebuffer to copy between them;
it can use a single target fbo and then render the source texture to
the FBO using a Cogl draw call; it can use a single FBO and call
glCopyTexSubImage2D; or it can fallback to reading all of the texture
data back to system memory and uploading it again with a sub texture
update.
Previously GL calls were used directly because Cogl wasn't able to
create a framebuffer without a stencil and depth buffer. However there
is now an internal version of cogl_offscreen_new_to_texture which
takes a set of flags to disable the two buffers.
The code for blitting has now been moved into a separate file called
cogl-blit.c because it has become quite long and it may be useful
outside of the atlas at some point.
The 4 different methods have a fixed order of preference which is:
* Texture render between two FBOs
* glBlitFramebuffer
* glCopyTexSubImage2D
* glGetTexImage + glTexSubImage2D
Once a method is succesfully used it is tried first for all subsequent
blits. The default default can be overridden by setting the
environment variable COGL_ATLAS_DEFAULT_BLIT_MODE to one of the
following values:
* texture-render
* framebuffer
* copy-tex-sub-image
* get-tex-data
