Windows: Make QFontMetrics(F)::ascent() and descent() consistent with other platforms

QFontMetrics(F)::ascent() and descent() on Windows currently returns the values retrieved by the system, which is not the actual typographic ascent/descent of the font, but the so-called winAscent/winDescent, which is used to calculate the maximum possible height of a line of text, not the typographic bounds.

This means (needs confirmation) that when we lay out several text lines on Windows, the distance between them may be larger than on macOS or Freetype, and it will be inconsistent with how the font was actually designed.

To make this more consistent in Qt 6, we will make the ascent/descent on Windows return the same as on the other platforms, i.e. the typographic metrics from the font data.