glsl: add ir_variable::determine_interpolation_mode() function.

This function determines how a variable should be interpolated based
both on interpolation qualifiers and the current shade model.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/glsl/ir.h

@@ -291,6 +291,17 @@ public:
     */
    const char *interpolation_string() const;
 
+   /**
+    * Determine how this variable should be interpolated based on its
+    * interpolation qualifier (if present), whether it is gl_Color or
+    * gl_SecondaryColor, and whether flatshading is enabled in the current GL
+    * state.
+    *
+    * The return value will always be either INTERP_QUALIFIER_SMOOTH,
+    * INTERP_QUALIFIER_NOPERSPECTIVE, or INTERP_QUALIFIER_FLAT.
+    */
+   glsl_interp_qualifier determine_interpolation_mode(bool flat_shade);
+
    /**
     * Delcared name of the variable
     */