Render fringe bitmaps correctly on NextStep.

The fringe bitmaps were inverted, the background was not
transparent, and the image data was flipped bitwise left-right.

* 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 it.
* nsterm.m (ns_draw_fringe_bitmap): Don't invert the image bits.