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GPU: add memory barriers for vertex and index buffers

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