#56 input.c not robust to multiple-display apps

[Bruce Merry]

input.c for X11 has a hack to get around the inability to query keysym<->keycode mappings in a callback, where keycodes are cached globally. However, if an application is connected to multiple X11 displays, which have different keycode<->keysym mappings, and calls Xlib functions on the different displays concurrently from multiple threads, the cache will be incorrect.