CaptureVertexPointerGLES1 was expecting only client-side vertex arrays
and not checking whether a vertex buffer was bound. This patch fixes
offsets in vertex buffers for GLES1, which can be seen in apps like
Dr. Driving.
The mClientVertexArrayMap index is now retrieved by entrypoint name, as
the arrayClientPointerIndex is unset in the vertex buffer case.

Bug: angleproject:5751
Change-Id: If40cae56bcdbed57885e83c49291cb000a884b0e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2859826Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Cody Northrop <cnorthrop@google.com>