Add support for creating Cairo brush from wxBrush with non-solid colour

Cairo colour patterns support only RGBA colours and therefore it is not
possible to create a colour pattern from wxBrush with non-RGB colour
(non-solid colour). However under wxOSX, when we have a solid wxBrush with
non-solid colour, we can convert it to Cairo surface pattern (stiple)
because under wxOSX non-solid colour is actually represented by a pattern
image.

src/generic/graphicc.cpp

@@ -1026,9 +1026,34 @@ wxCairoBrushData::wxCairoBrushData( wxGraphicsRenderer* renderer,
             InitStipple(brush.GetStipple());
             break;
 
+        case wxBRUSHSTYLE_SOLID:
+            // Non-RGB colours, which may e.g. appear under wxOSX for wxColours
+            // with NSColor backend, are not supported by Cairo and have to be
+            // handled in a special way.
+            if ( !brush.GetColour().IsSolid() )
+            {
+#if wxOSX_USE_COCOA
+                // Under wxOSX, non-solid NSColors are actually represented
+                // by pattern images and therefore a wxBrush with non-solid
+                // colour and wxBRUSHSTYLE_SOLID style can be converted
+                // to a stiple (a surface pattern).
+
+                // Create a stiple bitmap from NSColor's pattern image
+                wxBitmap bmp(brush.GetColour().OSXGetNSPatternImage());
+                InitStipple(&bmp);
+#else
+                wxFAIL_MSG( "Brush with non-solid colour is not supported." );
+#endif
+            }
+            break;
+
         default:
             if ( brush.IsHatch() )
+            {
+                wxASSERT_MSG( brush.GetColour().IsSolid(),
+                              "Brush with non-solid colour is not supported." );
                 InitHatch(static_cast<wxHatchStyle>(brush.GetStyle()));
+            }
             break;
     }
 }