When the vertex array binding is changed, the value of
ContextVk::mLastIndexBufferOffset must be reset so that an
old/cached offset (from a previous draw call) is not used for a
new draw call.

This addresses rendering artifacts and Vulkan validation errors
seen with running NBA 2K20 on Android (or the ANGLE trace on
Windows).

Bug: b/153682968
Bug: angleproject:4575
Bug: angleproject:4576
Change-Id: I69089964b8d278c7fc8610ab003548864a215dab
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2152850
Commit-Queue: Ian Elliott <ianelliott@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>