Bug fix #48: Premultiply icon image

 - Premultiply icon images from _NET_WM_ICON. _NET_WM_ICON (supposedly)
   provides non-premultiplied image data while X probably requires
   premultiplied ones. Absence of the conversation causes issues with
   transparent icons (for example, transparent parts become white).
   Thanks to Arkq and revast for reporting. (#48)

 - Use ARGB visual for XCreateImage() when depth is 32 when converting
   image data to pictures, instead of using the default (usually 24-bit)
   visual. In practice seemingly there's no effect, though. Presently
   it uses the result of find_argb_visual() from mainwin.c but maybe the
   initialization should be moved to skippy.c in the future.