Convert NS face colors to RGBA when comparing with frame values

Convert NS face colors to RGBA when comparing with frame values

The NS port uses indexes into a color table to specify the colors of
faces, whereas frames use RGBA pixel values.  In
extend_face_to_end_of_line the two needed to be compared to ensure
that the backgrounds of certain faces are not extended to the edge of
the window, which was failing because of this difference, thus causing
a visual difference with other platforms.  Convert from index to RGBA
when doing such comparisons.

* src/dispextern.h (FACE_COLOR_TO_PIXEL) [HAVE_NS]: New macro.  Call
ns_color_index_to_rgba under NS only.

* src/nsgui.h: Add prototype for ns_color_index_to_rgba.

* src/nsterm.m (ns_color_index_to_rgba): New function.  Converts a
color_table entry to corresponding RGBA pixel value.

* src/xdisp.c (extend_face_to_end_of_line): Call FACE_COLOR_TO_PIXEL
on face background color when comparing with frame color.