When we create a new font, we should store its height in a REAL field so
we don't have to round it. Further, when we calculate the width, we should
base the calculation on the metrics of a font at the height we will use, to
prevent rounding errors when the graphics transform will enlarge the font.

Like ca381b61, this was introduced by
f1ca14cb.

Don't overwrite the entire buffer if only part of it is not current. The data
we're copying may not be completely current either. This is for the case where
the inner rect is not current and the outer edge is. The reverse case where
the inner rect is current and we need to mask already works correctly.

msvcrt: Ensure that old buffer contents are copied when allocating a growable pf_output buffer for the first time.