In case fractional scaling is enabled, depending on the resolution,
mutter may advertise many fractional scaling values which are way more
than MAX_SCALE_BUTTONS, making impossible to use higher fractional
scaling values in high dpi setups.
So, in case scaling is possible, show them as buttons if they fit the
buttons limit, otherwise fallback to a combo box that is consistent with
the rest of the UI and will allow to use any valid scaling value.
This means we show the first 5 scales that are supported for the
display. That is the same behaviour as 3.30 had, therefore minimizing
the UI changes that users will see.
Note that there are plans to improve the scaling UI, however, it is not
yet clear how this will look like.
