Fix rounding issues of flat tuplet brackets

Issue #5615 (closed) happened because the offset of flat tuplet brackets was being set too strictly (to dy = 0 to be able to be considered flat) whereas optically they look flat even with values slightly different than 0. This led to an optically wrong collision detection and rounding issues which gave the impression that tuplet brackets don't vertically align.

Update (2nd commit): Issue #2190 (closed) happened because flat tuplet brackets that might collide with the extremal staff lines (1st or 5th) were not included in the staff span for collision checking.

Closes #2190 (closed)

Closes #5615 (closed)