refactor(font): More robust font size calculation

[patrick96]

Originally the size function returned the scaled size property for
scalable fonts and the non-scaled pixelsize property for non-scalable
fonts. This caused lots of issues when that property was 0 (empty bars,
characters not drawn without warning, see references at the bottom).
This behavior was mostly observed on debian where size is set to 0 if
pixelsize is set.

We now try to use both properties for both types, but prefering size
for scalable fonts and pixelsize for non-scalable ones.

This behavior doesn't break existing correct behavior but now never
returns 0. It will always try to fall back to the other property or to
some fallback value if both properties are 0.

I originally thought this could also make font patterns more expressive
by being able to specify the size of scalable fonts directly in pixels
like so:

Unifont:size=0:pixelsize=20

or to scale non-scalable fonts by forcing polybar to fall back to the
size property (which is always scaled):

Wuncon Siji:pixelsize=0:size=20

But how these two patterns are matched by fc-match depends both on the
font and on the distro/fontconfig setup.

Ref #706
Ref #1450
Ref #1257