Spritesheet Deep Copy Splicing
Changes the spritesheet dialog splicing to create deep copies of the
subimages so the original (usually much larger) spritesheet image is not
retained and becomes eligible for garbage collection. The technique used
was benchmarked to be quite fast using source composition which is
basically the same as a bitblt. This fixes the out of bounds errors in
the dialog because the subimage rectangle is intersected by the graphics
clip. The behavior is now the same as GM except for subimage rectangles
that lay on the edge or outside the spritesheet which will not contain
parts of the previous subimage (arguably a bug in GM). Later if the
editor were changed to use texture paging this would be needed anyway
to copy directly into the texture.
