Added love.graphics.isActive. love.graphics function calls (and method calls on objects created via love.graphics) are only guaranteed to work when isActive is true, otherwise bad things might happen (the program crashing, for example.)

It's usually only false when the app is inactive on iOS, and when the window hasn't been created yet.

--HG--
branch : minor

src/modules/graphics/opengl/wrap_Graphics.h

@@ -44,6 +44,7 @@ int w_reset(lua_State *L);
 int w_clear(lua_State *L);
 int w_present(lua_State *L);
 int w_isCreated(lua_State *L);
+int w_isActive(lua_State *L);
 int w_getWidth(lua_State *L);
 int w_getHeight(lua_State *L);
 int w_getDimensions(lua_State *L);