When an image was disposed, the image was not unregistered from the
set of 'imagesToMakeShared'. This caused a crash when trying to sync
the pixel data of the disposed image.
This change fixes the issue by unregistering the image when the
image is disposed.
Fixes#1421
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
