Permits all positive and zero-length buildings in skylines (issue 3161)

Permits all positive and zero-length buildings in skylines (issue 3161)

This allows point stencils to figure into vertical spacing, as shown
in the new regtest input/regression/skyline-point-extent.ly.

Some issues regarding floating point errors were raised while reviewing
this patch, but it was concluded that the math in this patch likely does
not risk triggering those errors with the way GCC compiles the code.