Set the EWMH _NET_WM_ICON window hint on X11 systems

Set the EWMH _NET_WM_ICON window hint on X11 systems

This patch implements window manager icons with 8-bit alpha transparency
on X11 systems.  The window manager must support the _NET_WM_ICON hint
described in the freedesktop.org Extended Window Manager Hints[*]
specification.

SDL does provide a function for setting the window manager icon
(SDL_WM_SetIcon), however, its transparency support is limited to a
simple bitmask describing which pixels are fully opaque and which are
invisible.

Note that the functionality implemented here should be ported to
SDL_WM_SetIcon at some point.

[*] http://standards.freedesktop.org/wm-spec/latest/

git-svn-id: https://s.snth.net/svn/neverball/trunk@2384 78b8d119-cf0a-0410-b17c-f493084dd1d7