Render fringe bitmaps correctly on NextStep (bug#21301).
The fringe bitmaps were inverted, the background was not transparent,
the image data was horizontally mirrored, and periodic fringe bitmaps
were not supported.
* nsimage.m ([EmacsImage initFromXBM:width:height:fg:bg:]): When
both background and foreground colors are 0, set the background
alpha channel to 0 (making the background transparent). When
copying the image data, do this from the most significant bit
(leftmost) to the least (rightmost), to avoid mirroring.
* nsterm.m (ns_draw_fringe_bitmap): Don't invert the image bits. Add
support for periodic images (e.g. the empty line indicator).