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`).
Reviewed By: fclem
Differential Revision: https://developer.blender.org/D13595
214a56ce8c