libgui|GLProgram: Ignore uniforms not present in the linked program

This facilitates use of common uniforms like uMapTime that are declared
in the generic shaders but may not end up being used, and thus are
removed at link time.

doomsday/sdk/libgui/src/graphics/glprogram.cpp

@@ -429,7 +429,12 @@ GLProgram &GLProgram::bind(GLUniform const &uniform)
 {
     if(!d->allBound.contains(&uniform))
     {
-        d->addBinding(&uniform);
+        // If the program is already linked, we can check which uniforms it
+        // actually has.
+        if(!isReady() || glHasUniform(uniform.name()))
+        {
+            d->addBinding(&uniform);
+        }
     }
     return *this;
 }