OpenGL 2.1 is now required to use love.graphics (resolves issue #779.)

Removed love.graphics.isSupported("canvas", "shader", "npot", "subtractive", and "mipmap"), since those features are all guaranteed by the minimum system requirements.

--HG--
branch : minor

src/modules/graphics/Graphics.h

@@ -78,13 +78,8 @@ public:
 
 	enum Support
 	{
-		SUPPORT_CANVAS = 1,
-		SUPPORT_HDR_CANVAS,
+		SUPPORT_HDR_CANVAS = 1,
 		SUPPORT_MULTI_CANVAS,
-		SUPPORT_SHADER,
-		SUPPORT_NPOT,
-		SUPPORT_SUBTRACTIVE,
-		SUPPORT_MIPMAP,
 		SUPPORT_DXT,
 		SUPPORT_BC5,
 		SUPPORT_INSTANCING,