Front-end: Warn for likely missed change in default precisions.

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).

Test/spv.structAssignment.frag

@@ -1,5 +1,7 @@
 #version 140
 
+precision mediump int;
+
 uniform sampler2D samp2D;
 in mediump vec2 coord;