This is part of the change to have desktop shaders respect precision
qualifiers on Vulkan, but since the defaults are all highp, and that's
different from ES fragment shaders, detect likely cases and warn about
them (but being careful to not be too noisy if it's unlikely to be a
problem).
Sets highp defaults for the appropriate types, for all stages,
and turns on precision qualifiers for non-ES shaders. Required
fixing some qualifier orders for desktop built-in declarations
for pre-420 shaders.
Before, it was only including explicit interface, sufficient for IO-Block-declared
oriented interface, but not sufficient for all modes GLSL might be used with
SPIR-V.
Two things are accomplished now:
1) each id will appear exactly once
2) the OpEntryPoint list will union static use with declarations
Structured control-flow rules allow leaving the middle of a construct through
a return, but not through a jump to a block that does a return.
Addresses issue #58.
