Issue 4689: Empty string at line spanner bound causes infinite loop
Empty stencils cause infinite loops when a line spanner tries to incorporate them into its calculations for its span_points. This is caused by two problems:
- line spanners could be created with infinite span_points
- make_trill_span will loop endlessly if passed infinite values
Check for and skip empty stencils when incorporating left and right stencil extents into a line's span_points. Check for infinite offsets in make_trill_span. Add a regression test.
Closes #4689 (closed)