Reinstate GDI+ renderer for 32bpp images (when zoomed out)

(Relevant test image:
http://www.renders-graphiques.fr/galerie/Objets-65/Ballon-Multicolore-B-116730.htm)

The recent viewport overhaul has made it trivial to implement different
viewport renderers.  In this commit, I brought back GDI+ as a rendering
option for zoomed-out 32bpp images.

AlphaBlend is a nice API call, and reasonably fast, but it only offers
nearest-neighbor interpolation.  When zoomed out, this results in
sub-par image quality.

GDI+ offers a host of different interpolation options, making it a nice
choice for 32bpp rendering.  This commit specifically uses a low-quality
bilinear filter.  It's slower than AlphaBlend - no surprise there - but
the resulting viewport is much prettier, and much more representative of
the actual image.

Ideally, there would be a preference for viewport render quality vs
speed.  I don't want to work on this just yet, though, as the viewport
will be getting a lot of changes in 6.4 due to layers.  This update is
mostly to see if I get any complaints re: performance in 6.2, though to
be honest, 32bpp images large enough to require zooming-out are not a
common use case...