LibGfx: Add paint styles and allow gradients to be used as them

Also while here add option to disable pre-multiplied alpha for gradients
(this will be handy later).

Userland/Libraries/LibGfx/Forward.h

@@ -24,6 +24,7 @@ struct FontPixelMetrics;
 template<typename T>
 class Line;
 
+class AntiAliasingPainter;
 class Painter;
 class Palette;
 class PaletteImpl;