We now check for sampler overflow during material compilation rather
than waiting for run-time checks. This allows for Kokoro-based
validation of sample materials, and allows developers to catch this
issue in their asset pipeline.
We should also probably raise the upper limit (Qualcomm allows up to 16
samplers in their Vulkan implementation) but that can be a separate PR.
Fixes#507
prideout
cfa9895b96