Add TODO comment.
Separate defaut scale to constant variable outside.
Implement removing tile.
Implement scroll tiles.
Merge branch 'next'
Move cursor from [front] of tile to [center] of tile.
Separate calculation and drawing.
Eliminate duplication of drawing.
Separate execution of calculation and that of drawing.
Implement magnify.
Merge branch 'next'

Conflicts:
	sdlgui.cpp