glsl: Cross validate variable's invariance by explicit invariance only
invariant qualifier is propagated on variables which are used
to calculate other invariant variables, however when we are matching
variable's declarations we should take into account only explicitly
declared invariance because invariance propagation is an implementation
Thus new flag is added to
ir_variable_data which indicates
qualifier being explicitly set in the shader.
explicit_invariant flag is a strict subset of
I don't really like the addition of new flag but in the end it looks clean enough.
Moved patch from mailing lists where was getting dusty.