Gene symbol visibility when zooming in close enough

[tuner]

The fancy semantic zooming is wonderful ... until one zooms close enough. The gene symbol disappears:

Where.s.the.symbol.mov

The disappearing gene symbol hasn't been an issue so far, as we have been mostly interested in relatively large aberrations, such as copy-number segments, where the exact breakpoint has not been that important. This changes now that we are working with structural variants.

The problem could be solved by using ranged text. But ... the arrow (a "point" mark) would get detached from the symbol text. Implementing secondary coordinates for point mark solely for this use case would be a way too ugly hack.

This could probably be solved by using some transform (like #160) that adjusts the linearized genomic coordinates so that the data objects stay within the viewport, behaving similarly to ranged text.

Edit: As text lengths are measured in pixels, this, in practice, needs ranges to be specified as pixels. Now we have unit ranges [0, 1]. #205 and #207 will help here.