* Enable optimization passes on materials with external samplers
Because external samplers are not properly supported by SPIRV and
associated library (spirv-cross and spirv-tools) we currently disable
all optimizations when we encounter a material with external samplers.
This however causes issues on some misbehaved drivers (not running
the optimizations has a side effect which causes a crash). To enable
the optimization pass we simply rely on the Vulkan codegen target to
substitute samplerExternal with sampler2D. We then analyze the output
GLSL (post-optimization) and revert the relevants sampler2D declarations
to samplerExternal declarations.
This fixup only occurs after optimization and for mobile targets and
if external samplers were declared.
* Address review comments
e41dd027ab