acme: fix window and scrollbar display glitches at bottom fringe of column

The following patch fixes acme display glitches at the bottom fringe
of columns when adding/moving/resizing windows.

Here an example of an easy to reproduce case:

• https://invidio.xamh.de/watch?v=iLekQrxycaM

…opening acme and resizing a column to the right is all that is needed.

The functions winresize(…) and textresize(…) are extended with an
additional parameter `fillfringe` to indicate if a window/tag shall
fill a potential fringe area that would otherwise remain white.

The changes have been inspired by the approach taken in plan9port
acme.

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