In this commit, VertexArrayVk::convertVertexBuffer() is renamed to
VertexArrayVk::convertVertexBufferCpu() to explicitly show it does a CPU
readback.  A new VertexArrayVk::convertVertexBuffer() function is added
that has the same functionality in gpu (with some assumptions, where the
CPU fallback is used should those assumptions fail).  Currently, the
only requirement is that buffer offset/stride are divided by the
component size.

ConvertVertex.comp is the shader responsible for this conversion, and it
implements the functionality in renderer/copyvertex.inc, minus a few
functions that are not used in the Vulkan backend.

Bug: angleproject:2958, angleproject:3009
Change-Id: I8ec9a5f4672509bcf7b9e352cd27663970ad4653
Reviewed-on: https://chromium-review.googlesource.com/c/1364451
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>