improve visibility of overlapping annotations

[bwl21]

problem

when annotations are overlapping (e.g. barnumbers, countnotes) none of the two are readible.

solution approach 1

• [x] improve accuracy of size of annotations
• [x] draw a white background
  • [x] implement a function to create a white rectangle from Annotation
  • [x] compute backgrounds at the end of layout_voice

solution approach 2

• [ ] improve accuracy of size of annotations
  • [ ] handle multilines
  • [ ] use character widths instead of a heuristic factor