GtkGLArea: Only re-allocate buffers during paint
This drops the maybe_allocate_buffers that re-allocates buffers at any point. Instead we just set have_buffers to FALSE and have the buffers re-created when needed. This also makes the buffer creation code imdeponent and makes it clean up no longer needed buffers in order to handle being called multiple times due to the above. We also ensure we re-allocate the buffers when we're resizing and the buffers are already created.
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