Vulkan: Use storage buffers for index conversion shader. This allows us to use the shader regardless of driver support for texel buffer views. It also allows us to convert indices on the GPU unconditionally. We add a new internal compute shader that converts pairs of indices into a packed single uint value that stores two 16-bit values. In the future we could add support for converting primitive restart indices. Should speed up benchmarks on systems which didn't have R8_UINT support for compute shader buffers. Bug: angleproject:3490 Change-Id: I56ca0cabb094e97f36ab4edc779e6c8ad2d2601e Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1639058 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com>