Now grpahicscommand saves the error and shows the error after a
while. This was good to simplify the API but was the cause to hide
some issues.
This change fixes all the errors to be returned immediately, and
buffer this in the ebiten package instead.
Fixes#971
Before this change, it was not clear that shareable's Dispose can
be called from finalizers since finalizers must not be blocked by
a mutex. Actually Dispose could be locked and must not be called
from finalizers.
This change renames the function to avoid confusion of the API,
and make the function available from finalizers.
