Gem Drop X now uses 100% SDL

Last night I converted all of the Xlib calls in my game “Gem Drop X” to
SDL calls. So, the game is 100% SDL.

(Note, it still uses Linux stuff for joystick support, so it should be
portable to other OSes, minus joystick support.)

Gem Drop X is based directly on the Action! language source code to an
Atari 8-bit game I wrote about two years ago, called “Gem Drop.”

The concept is based on what I saw of the SNK NeoGeo arcade game
"Magical Drop III."

It’s been compared to Tetris, Klax and Space Invaders.