Changes

select: calc: Use pre-defined F_100 macro

select: unit: angle2deg: Use pre-defined F_180 macro

select: computed: Wrap code to fit 80 cols

API: computed: Change css_computed_width getter function names
Now `css_computed_width()` is unchanged from how it always was. And
`css_computed_width_px()` is the new function to convert to device
pixels.
Also improve `css_computed_width_px()` API documentation.