Added "lighten" and "darken" blend modes (resolves issue #443).

They perform the equations res.rgba = max(dst.rgba, src.rgba) and res.rgba = min(dst.rgba, src.rgba) respectively.

They only work when the "premultiplied" blend alpha mode is used, and an error occurs otherwise.  They also aren't supported on some non-iOS OpenGL ES 2 systems. Check for support with love.graphics.getSupported().lighten.

src/modules/graphics/Graphics.h

@@ -84,6 +84,8 @@ public:
 		BLEND_ADD,
 		BLEND_SUBTRACT,
 		BLEND_MULTIPLY,
+		BLEND_LIGHTEN,
+		BLEND_DARKEN,
 		BLEND_SCREEN,
 		BLEND_REPLACE,
 		BLEND_MAX_ENUM
@@ -138,6 +140,7 @@ public:
 	{
 		SUPPORT_MULTI_CANVAS_FORMATS,
 		SUPPORT_CLAMP_ZERO,
+		SUPPORT_LIGHTEN,
 		SUPPORT_MAX_ENUM
 	};