Fix strange GDI+ premultiplication issues when generating thumbnails

GDI+ resizing does odd things to image edges when doing drastic resizes
(as when creating thumbnails), and it can screw up alpha effects.  To
remedy it, we now toggle premultiplication to force a reset of all alpha
values.
Implement click-drag scrolling for the image tabstrip

Now I can finally get rid of that hideous horizontal scrollbar.  :)
Actually, it still exists, as it provides an easy way to track and allow
scrolling - but it is kept invisible.

This is also a nice bonus for touchscreen users!

The one downside to this approach is that it's not immediately obvious
that the list can be click-dragged... but I'm not sure how to avoid
that.