Inlines a number of Vulkan vertex array methods.

Also changes the way vertex buffers are bound. Note that Vulkan doesn't
support NULL buffer bindings. Thus we create an emulated NULL buffer
to work around the problem of having gaps in the bound vertex buffers.
This allows us to use a single bind call for ranges of vertex buffers
even when there are gaps.

Also changes how vertex array dirty bits are reset. Instead of calling
memset to clear the affected buffers we pass a mutable pointer to the
Vertex Array sync state. This allows us to only reset the dirty bits
that we sync. This saves on the memory clearing time.

Improves perf by about 10% in the Vulkan VBO state change test.

Bug: angleproject:3014
Change-Id: Ib7b742dff7897fc891606a652ea0b64255a24c86
Reviewed-on: https://chromium-review.googlesource.com/c/1390360
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>