After Fill command, the image doesn't have to keep the restoring
information. Now Fill command is as same as DrawTriangles, there
is no way for restorable.Image to know whether it can reset the
state or not. ResetRestoringState clears the state explicitly.
Also, all the images cannot use the shareable backends before the
game starts, but it is expected that read-only images will use
shareable backends later.
Fixes#879
