[Viewer] Lock font height to 16 pixels

We're going to display gaiji characters as well, and scaling them to any other
size won't be pretty.
[Viewer] Automatically scroll to SS:SP when switching to Stack mode

Finally, this also works when the emulation is paused. Really, in the 1 ½
years the stack viewer has existed, I've *never* found myself in the situation
where I would have used the stack view for anything *other* than the actual
stack.