Issues with Scintilla for Qt

Qt reports character descenders are 1 pixel shorter than they really are.
There is a tweak in the code to add a pixel in. This may have to be reviewed for Qt 5.
There's a comment in the Qt code for Windows:
       // ### we subtract 1 to even out the historical +1 in QFontMetrics's
       // ### height=asc+desc+1 equation. Fix in Qt5.

The clocks used aren't great. QTime is a time since midnight clock so wraps around and
is only accurate to, at best, milliseconds.

On macOS drawing text into a pixmap moves it around 1 pixel to the right compared to drawing
it directly onto a window. Buffered drawing turned off by default to avoid this.
Reported as QTBUG-19483.

Only one QPainter can be active on any widget at a time. Scintilla only draws into one
widget but reenters for measurement.