* When calculating the height of text for vertical alignment purposes,
don't use the minimum height but stick to ascent + descent - like the old
renderer. This gives Text items a constant height that depends on the
font size and not the content.
* Avoid scaling text metrics twice. It turns out that femtovg uses the
device pixel ratio / dpi argument for `set_size` only for scaling text
metrics. Since we do all the mapping from logical pixels to physical pixels
it's correct to let femto only operate in physical pixel range. As a bonus
this allows removing the refresh_window_scale_factor() dance in the
backend (including making the context current, etc.)
