Clean up debug messaging during image loading
GDI+ interface: new function for converting 24bpp data to 32bpp

Previously this was always done manually, but I found a way to "coax"
GDI+ into doing it for us.
Image loading: use GDI+ to convert 24bpp images to 32bpp

PD auto-converts all incoming images to 32bpp.  For very large photos,
the 24->32bpp conversion step requires a non-trivial chunk of time.

A new GDI+ function (added in the previous commit) will now be
preferentially used in place of PD's manual conversion code, cutting
conversion time.

As a quick example, on my old Win 7 desktop with a folder of 25 photos
ranging from wallpaper size to 10mp, the GDI+ approach cut total loading
time from 22 seconds to 18 seconds - an 18% improvement.