Fixed up the API: setCanvas (singular) now only takes one canvas, setCanvases must be used for multi-canvas rendering

--HG--
branch : MRTs

src/modules/graphics/opengl/wrap_Graphics.h

@@ -90,6 +90,7 @@ int w_getPointStyle(lua_State *L);
 int w_getMaxPointSize(lua_State *L);
 int w_newScreenshot(lua_State *L);
 int w_setCanvas(lua_State *L);
+int w_setCanvases(lua_State *L);
 int w_getCanvas(lua_State *L);
 int w_setShader(lua_State *L);
 int w_getShader(lua_State *L);