BufferVk::mHostVisibleBufferPool is allocated when BufferVk::mBuffer is
not hostvisible and we need to map it. In that case
mHostVisibleBufferPool is allocated and data copied from mBuffer to it
and the pointer to mHostVisibleBufferPool is returned to user.
BufferVk::mMapInvalidateRangeStagingBuffer is used when map is called on
a small range. In this case we allocate memory for the small range of
buffer and return that intead of waiting for entire buffer for GPU to
finish. Also when BufferSubData is called, we also needs to allocate a
staging buffer and issue a copyBuffer from staging buffer to main
buffer. This CL consolidate all these three usage cases into one
mStagingBuffer. It removes mHostVisibleBufferPool and
mMapInvalidateRangeStagingBuffer from BufferVk class. This makes overall
logic of managing data consistency much simpler as well since we only
have two buffers: The main buffer storage mBuffer or mStagingBuffer. And
mIsStagingBufferMapped tracks if mStagingBuffer is the one actually
mapped to user or not so that at unmap time we know if we should flush
the data to mBuffer or not.

Bug: b/208323792
Change-Id: I4f0c79a2d86da1a43844ed2ba83ddeb7dd4a5c0b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3335216


Reviewed-by: Lingfeng Yang <lfy@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Charlie Lao <cclao@google.com>