BM_XRGB doubles its inputs. 35a4736 doubled the inputs explicitly, but
passed them back to BM_XRGB, which doubled them again. Switch to a
direct call to gr_find_closest_color to skip the second double pass.
Reported-by: Mako88 <https://github.com/dxx-rebirth/dxx-rebirth/issues/192>
Fixes: 35a4736df0 ("Pass color to gr_urect")