Moved font cache code to seperate source file
Removed X11 font pixmap reference counting

Font pixmap created/destroyed by sgui_init/deinit
Added parent window parameter to X11 pixmap

  - Pixmap window specific as intended
  - Can still pass root window
Added A8 (8 bit alpha) color format
Fixed pixmap load bug

  - Parameters were swapped in function definition
  - Clipping code used wrong coordinate
Made X11 font cache use the normal pixmap & canvas API