The intent of this CL is to call convertVertexBuffer*PU only when we
have new data to convert. If the app unbinds and rebinds a vertex
buffer without changing the data we can now retrieve the cached
vertex buffer info from the BufferVk class. Previously we would always
reconvert the data on a rebind. This was slowing down applications and
benchmarks.

To achieve this we add a conversion cache to BufferVk. Each cache entry
stores a key based on the vertex info. Also we store a ring buffer for
each cache entry and a flag to indicate if the entry is dirty. The
cache is dirtied on a bufffer data update or a map call.

Improves performance in the T-Rex benchmark.

Bug: angleproject:3495
Change-Id: Ia999c9187510748ba95bc98362eb332e1990d270
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1638903
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>