glsl: Introduce a new "const_in" variable mode.

This annotation is for an "in" function parameter for which it is only legal
to pass constant expressions.  The only known example of this, currently,
is the textureOffset functions.

This should never be used for globals.

src/glsl/ir_print_visitor.cpp

@@ -97,7 +97,7 @@ void ir_print_visitor::visit(ir_variable *ir)
    const char *const cent = (ir->centroid) ? "centroid " : "";
    const char *const inv = (ir->invariant) ? "invariant " : "";
    const char *const mode[] = { "", "uniform ", "in ", "out ", "inout ",
-			        "sys ", "temporary " };
+			        "const_in ", "sys ", "temporary " };
    const char *const interp[] = { "", "flat", "noperspective" };
 
    printf("(%s%s%s%s) ",