improve visibility of overlapping annotations

[bwl21]

problem

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

solution approach 1

• improve accuracy of size of annotations
• draw a white background
  • implement a function to create a white rectangle from Annotation
  • 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