Shaders: minor cleanup.

src/Magnum/Shaders/Flat.frag

@@ -165,6 +165,10 @@ in lowp vec4 interpolatedVertexColor;
 flat in highp uint interpolatedInstanceObjectId;
 #endif
 
+#ifdef MULTI_DRAW
+flat in highp uint drawId;
+#endif
+
 /* Outputs */
 
 #ifdef NEW_GLSL
@@ -181,10 +185,6 @@ layout(location = OBJECT_ID_OUTPUT_ATTRIBUTE_LOCATION)
 out highp uint fragmentObjectId;
 #endif
 
-#ifdef MULTI_DRAW
-flat in highp uint drawId;
-#endif
-
 void main() {
     #ifdef UNIFORM_BUFFERS
     #ifdef OBJECT_ID