wingdi: fix handling of 256 color depth
On Windows, 256 color bitmaps always have color palettes. Prior to this
change, we wrote out 8bpp RGB bytes where GDI instead expected 8-bit
palette indexes. Worse, we did not allocate memory for the palette, and
so GDI would read palette entries outside the end of the
vout_display_sys_t
struct. Since GDI will happily convert to and from
bitmaps of differing depths, we now write out a 24bpp bitmap for 256
color screen depths and let GDI convert to the paletted format.
In this commit, we also implement a similar fix for GDI screen capturing.