... 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.
Create WaveTrackCache as a utility class but don't use it anywhere yet.
The possible minor performance problem with effects is fixed by changes
in WaveTrack::GetBestBlockSize().
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.
