css: always get default font size in pixels

css: always get default font size in pixels

Fixes a couple bugs...

 - Pixel font sizes in css would render as point sizes.

 - For em font sizes, where the parent size was set and not default, we would
   incorrectly convert a pixel value from points to pixels.

We'll always grab the default font size in pixels so we don't keep confusing
things.

Worth noting that gtk css font-size will still behave differently than the
web. Pango interprets font-size differently.