This is in order to make the API more multithread friendly inside the
draw manager.
GPU_shader_get_uniform will only serve to query the shader interface and
not do any GL call, making it threadsafe.
For now it only print a warning if the uniform was not queried before.
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
