+ Internally cache screen width and height.
  * src/bitmap.c:
    + Added alpha support to caca_draw_bitmap(). For now, we only treat 0%
      alpha as fully transparent, and any other value as fully opaque.
    + Slightly tuned colour weights in the renderer.
    + caca_set_bitmap_palette() takes unsigned ints.
  * examples/demo.c:
    + Added a crap render demo.
  * examples/aafire.c:
    + Set a 20ms delay.
    + Added alpha values to the palette.
  * examples/view.c:
    + Draw a gray checkered grid below the image so that transparent images
      look a lot nicer.
    + 'f' toggles "fullscreen" mode.

+ Removed unused USE_* variables.
    + Export -lslang and -lncurses as @CACA_LIBS@.
  * examples/aafire.c:
    + Imported aafire.c from the AAlib tree and ported it to libcaca.