Fix image tabstrip error when invalid image is loaded.
Temporarily disable GDI+ support in the viewport renderer

I'm getting some oddly unpredictable results on 32bpp images while
zoomed out, and as I'll soon be rewriting the viewport to account for
premultiplied alpha in all layers, I don't want to waste time debugging
it just yet.

The viewport still works just fine; it just uses the old GDI-only code
path.
Add a bit of helpful debugging code to the window manager

Prompted by a discussion at vbforums on subclassing messages related to
Aero Snap, on which I have recently become an expert... (rolls eyes)
Greatly improve GDI+ image load function

The original version of this function was the first bit of GDI+ code I
ever wrote, and it was not pretty.  (Images were passed back to the DIB
via StdPicture!  Ouch!)  The new codepath uses a year's worth of
accumulated GDI+ knowledge to greatly improve performance for both 24
and 32bpp images, including relying on GDI+ itself to unpremultiply
alpha as necessary.  (In the next few weeks, I will toggle this to apply
premultiplication, which is part of why I wanted to rewrite the code in
advance!)

This was also a good opportunity for me to test LockBits, which works
very well for copying large amounts of raw bitmap data between GDI+
objects and DIBs.