GPUFrameBuffers were being free when no context was attached or in the
wrong gl context. This make sure this does not happen again.
You can now safely free any gl resource from any thread (well as long as
it's not used anymore!).
This does not include all the struct and type renaming. Only files were
renamed.
gwn_batch.c/h was fusioned with GPU_batch.c/h
gwn_immediate.c/h was fusioned with GPU_immediate.c/h
gwn_imm_util.c/h was fusioned with GPU_immediate_util.c/h
