VideoInterface: accurate XFB width and height.

What we call "fbWidth" here is really the stride, but I'll fix that in a
followup.
VideoCommon: recreate XFB texture when the XFB size changes.

We need to do this to correctly deal with games which dynamically change the
XFB width and height.
VideoInterface: clean up naming and bitfields.

Matching the hardware more closely will hopefully make this code easier to
read.
Merge pull request #336 from magumagu/xfb-vertical-scale

XFB width/height handling fixes