- solidify/solid.h: definition of a solid made from two opposing faces
- solidify/overlap.h (overlap), solidify/overlap.c (draw_image,
scroll_event, overlap): operate on solid instead of face
- solidify/overlap.c (sx, sy, draw_image, overlap): helper functions sx()
and sy() to determine canvas size
- solidify/overlap.c (BORDER, sx, sy): added a border around the piece,
to help with positioning
- solidify/solidify.c (clicked, gui_buttons, gui, main): use solid instead
of faces
