Fix subtitle scaling for text based subs during windowed video playback

This implements scaling.  It's not perfect, and there's a couple of simple efficiency things that could be explored (e.g. the transform is trivial to write down directly as one of the translations can be dropped, so no need for the matrix computations).

Basically it moves from screen coordinates back to control coordinates, but takes into account the fact that we don't want to scale by the control coords -> screen coords scale factor (as fonts are rendered in the screen space anyway).

A little convoluted, but when we have screen coords to play with, and the font stuff operates in control coords, it was the only way I could see of doing it without doing lots of changes to the font rendering stuff.

Obviously the confluence change needs dropping before this is pulled in.