Convert fringe bitmaps to vectors on NS port

Convert fringe bitmaps to vectors on NS port

Unfortunately *step doesn't support masks for bitmap images so
changing the colors of fringe bitmaps is awkward.  We can work around
this by converting the bitmap into an NSBezierPath and drawing it in
the required color.

* src/nsterm.m (ns_define_fringe_bitmap):
(ns_destroy_fringe_bitmap): New functions
(ns_draw_fringe_bitmap): Display the NSBezierPath.
* src/nsimage.m
([EmacsImage initFromXBM:width:height:fg:bg:reverseBytes:]): Remove
variable that's there to allow us to easily modify the XBM colors.
([EmacsImage setXBMColor:]): Remove method.