Speedup in AlbTextEditorLineSegmentPieceMap

[tinchodias]

This optimizes a bit dragging the scroll bar.

See: #34

In my informal profile measurements, I saw a 2x speedup in selectionRectangles , still slow.
I enclosed this calculation in a [ ] timeToRun and log shows between 5ms and 12ms on each execution. Probably we can achieve 0ms by caching. Still understanding what's the motivation of this selectionRectangles. Solutions may be caching the result, or count with the inverted Dictionary that it now problematic (one where we search over the keys of it, not the values).

Maybe @plantec has an idea

[tinchodias]

After mocking selectionRectangles ^ #() I see more reasonable profile results...
10% layout
13% measuring text extent via Cairo
28% drawing with Cairo
Bah... at least FFI starts to be more relevant in the profiler result, I think that makes sense as for the moment we are redrawing each time. Anyway if we do a step back, we could avoid measuring text and redrawing if it really didn't change. On this could help pharo-graphics/Bloc#268

[tinchodias]

@plantec I marked it "WIP" (work in progress) because I wanted to push more commits before merging... but finally didn't do more. Do you agree to merge it, isn't it? it 's a refactoring that speedsup.