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
Mipmap calculation must be executed after the main loop starts
because the graphics driver's HasHighPrecisionFloat is needed.
Then, operations on mipmap images must be called from images in
buffered package.
Updates #1044
