This adds memory barriers to use with GPU_memory_barrier to ensure that
writes to a vertex or index buffer issued before the barrier are
completed after it, so they can be safely read later by another shader.

GPU_BARRIER_VERTEX_ATTRIB_ARRAY should be used for vertex buffers (GPUVertBuf),
and GPU_BARRIER_ELEMENT_ARRAY should be used for index buffers (GPUIndexBuf).