Implement BufferVk::copySubData().

Bug: angleproject:3194
Test: dEQP-GLES3.functional.buffer.copy.*
Test: dEQP-GLES3.functional.negative_api.buffer.copy_buffer_sub_data
Test: angle_end2end_tests BufferDataTestES3
Change-Id: I220cd490eb1eb799604e217b3e377af3fd431d97
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1648669
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>