The application Blitzin2 was sending WM_VSCROLL messages to the
richedit control directly, when normally this message is supposed to
be a notification sent after the scrollinfo is set.  Native richedit
controls always use the 16-bit value passed to this message to set the
scroll position for SB_THUMBPOSITION, rather than trying to find a
32-bit value through GetScrollInfo like I had previously done.

This is the same pointer, but lastActiveRenderTarget will go away.

Once we keep track of the wined3d context for each thread,
lastActiveRenderTarget won't do what we want here.

The missing return was to prevent automatic vertically scrolling when
style for it is not set, but is set for the horizontal scrollbar. I
initially missed this by testing with no autoscrolling on either
scrollbar.