ClipToPicture test: Fixed nested states expected look.
app_server: Implemented nested clipping paths

 * The alpha mask is no longer stored with 75% more memory than needed,
   resolving a TODO.
 * AlphaMasks are now BReferenceables.
 * AlphaMasks are transferred to a pushed DrawState.
 * When an AlphaMask is set on a DrawState, it sets the previous state's
   mask to the AlphaMask. That one now takes care of updating not only itself,
   but the previous mask as well (which works recursively).
 * In AlphaMask::Generate(), a combination happens with the previous state's
   mask, which again works recursively in case the previous mask also needs
   to be updated. This step is combined with extracting the alpha channel from
   the UtilityBitmap used to play the ServerPicture.
 * Fixed some out of bounds access to memory in the "outside" case in
   agg_clipped_alpha_mask.h. This happened when the requested region was
   both before and after where the mask has data.
agg_clipped_alpha_mask.h: Partially revert previous commit.

There wasn't an out-of-bounds access, I read the code wrongly. Keep the code
in sync with the AGG version. What happens is that for x < 0 and xmax < 0,
the code will memset the x < 0 region twice, unless I am still irritated...