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>