This is a small change. We delay all gl calls at the first use of the
GPUIndexBuf / GPUVertBuf in order to be able to create multiple buffers
from different threads without having many gl contexts.
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