Various: fix MAX_LONG value to be &HEFFFFFFF instead of &HFFFFFFFF

Leading bit is the sign bit, so &HFFFFFFFF = -1.  I thought I'd fixed
all these occurrences, but a few escaped detection.
pdUndo: change default thumbnail sizes
pdUndo: remove unnecessary temporary DIBs

We can create the thumbnails straight into the cache, without a
temporary DIB.
Undo history: fix potential bug with thumbnail rendering

Related to recent pdUndo thumbnail size changes - my bad.
Effects > Artistic > Stained Glass: initial build

I've been eying Voronoi-based effects for awhile, and RR's recent work
on a similar tool encouraged me to take the leap.  Many thanks to him
for discussion and testing on this topic - it was invaluable in avoiding
potential pitfalls, and implementing everything properly from the start.

Relevant reading for understanding how PD's technique works:

http://en.wikipedia.org/wiki/Voronoi_diagram
http://en.wikipedia.org/wiki/Worley_Noise

The new pdVoronoi class handles most the heavy lifting, and in a rare
development for this repo, the class is completely standalone (e.g. it
has no dependencies on other PD code).  Hopefully this means others can
make use of it, though the currently available functions are heavily
tailored toward my PD uses (rather than an all-purpose Voronoi
calculator).

I've got a lot more refining to do on this tool, as you can see from the
UI and code, but as it's fully functional and quite well-optimized, I
want to take a snapshot before diving deeper.

Despite its complexity, performance is already quite good - real-time
previews in the IDE, and 8mp photos are processed in ~3 seconds on my
old Win 7 desktop.