The current caching logic stores multiple static attributes in a single
buffer if the attributes share a D3D buffer - sometimes.
If a buffer is 'committed' (has been used in a draw) then we would
make a new D3D buffer for the next set of static attributes.

Instead use a simpler scheme of a single D3D buffer for each
static attribute. Also change rx::VertexBuffer to a reference
counted class. This simplifies the caching logic for static vertex
buffers (translated attributes) considerably. We only need to
release the buffers when the ref count is zero, and ensure we
track the ref count correctly when bound to D3D.

This leads the way towards using a simpler dirty bit scheme for
intelligent state updates, and less overhead doing work with
buffer state updates.

BUG=angleproject:1327

Change-Id: I99461d50b9663024eaa654cd56b42a63f1416d08
Reviewed-on: https://chromium-review.googlesource.com/330170Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>