... Should have no effect on generated code, except perhaps some slight faster
virtual function calls. Mostly useful as documentation of design intent.
Tried to mark every one of our classes that inherits from another, or is a
base for others, or has abstract virtual functions, and a few others besides.
... and SelectedRegion, and not ViewInfo or zoom level as a double.
Also some leftOffset arguments.
Assumptions of uniform zoom level persist in TrackArtist::DrawClipSpectrum and
in TrackArtist::DrawClipWaveform but no longer in the rest.
When it is enabled, the project can scroll up to one-half of a screenful
beyond time zero or the maximum track time. I was careful to disable selection
of negative times.
This is motivated by the smooth scrolling scrub. It behaves more sensibly at
the extremes. It can still keep the play indicator centered.
Also removed an unused member of ViewInfo.
This change is believed to be a direct refactoring that does not change functionality. It paves the way for more complex kinds of selection, such as selections involving frequency as well as time. It also reduces risk of left and right edges being swapped in future code using SelectedRegion, as the default is to swap on assignment if needed.
