Integrate make-frame-stencil (makeDeltaSpan) and makeDeltaFrame

[uliska]

It is unacceptable to have these more or less redundant functions side-by-side.

The solution is:

• have one function make-frame-stencil
• Draw the border like in makeDeltaFrame
• Only draw the inner polygon if color is not ##f'

This will also make it easier to have the border grow outwards and (maybe) to have the inner polygon not rounded if there is a border.

[KlausBlum]

Done. New branch "integrate-MakeDeltaFrame" is ready for revision.

I could not completely remove the double execution of calculations, because the inner polygon needs a different order (Points must be given in clockwise order - TODO: Test if this is really necessary). At least, calculations are only executed if a polygon will be drawn.

Now it is possible to have "empty" borders without inner polygon. HOWEVER, border-radius > 0 will only lead to usable results if there is no border (i.e. only inner polygon). Therefore I would recommend to set the default value to zero. This also prevents the problems with broken frames or when working with open-on-top / open-on-bottom.

Okay... I'm open to opinions and suggestions.

[KlausBlum]

Update: No need to stick to clockwise order. With the lastest commit, all redundant code is removed.

[KlausBlum]

Another update: With the latest commits, it's again possible to have border-radius > 0. ly:round-filled-polygon has an optional parameter extroversion that prevents the ugly distortions when set to zero. Nevertheless, I'd still recommend to have zero as default value.

Any thoughts?

[KlausBlum]

Merged 21.06.2020