Although renderImage for ImageId doesn't use _currentImage, it needs to
be set for compatibility with the section based renderImage function.
The functionality doesn't change with this commit, except that the
palette brightness gets updated if the image is in a different file.
Although SupernovaEngine forwards render calls to screen, it also
exposed members publicly like _brightness. Therefore, a few changes were
necessary in rooms and state.
Joseph-Eugene Winzer
Adrian Frühwirth