This obviates many static_casts. More closely represents what Vulkan
is doing. One cast remains where we are using uint32_t for dynamic
offsets with uniform buffers.

Bug: angleproject:2786
Change-Id: I043ebb5d965f46ee6ebbfcb91d75a58508cacfe3
Reviewed-on: https://chromium-review.googlesource.com/1188949Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>