Added 'VertexBuffer' abstraction. SpriteBatches are now always supported.

Also improved memory usage in SpriteBatch. (No need to retain full copy of
sprites in memory. Can read that data back later to save data from OpenGL
context destruction).

src/modules/graphics/opengl/wrap_Graphics.cpp

@@ -774,10 +774,6 @@ namespace opengl
 					if (!PixelEffect::isSupported())
 						supported = false;
 					break;
-				case Graphics::SUPPORT_SPRITEBATCHES:
-					if (!SpriteBatch::isSupported())
-						supported = false;
-					break;
 				default:
 					supported = false;
 			}