Make Snake, Tetris and Pong adjust to display DPI

Make Snake, Tetris and Pong adjust to display DPI

Replace gamegrid's constant 16x16 glyph with a generated one based
on display dimensions (bug#24658).
* lisp/play/gamegrid.el (gamegrid-glyph-height-mm): New variable.
(gamegrid-glyph-height): Deleted.
(gamegrid-xpm, gamegrid-xbm): Constants replaced with functions.
(gamegrid-colorize-glyph): Use new functions instead of constants.
(gamegrid-calculate-glyph-size, gamegrid-insert-xbm-bits): New
functions.