This make package shareable more consistent. The mutex is lock
after EndFrame and before BeginFrame, and the similar rule will be
applied at launching (BeginFrame unlocks the lock in any cases).
Instead, package ebiten queues image operations if BeginFrame and
doesn't create provisional non-shared images. This should improve
performance at launching since this reduces the number of draw
calls, especifally for creating new images.
Updates #879.
Updates #921.
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
