Use a dumb cache for waveview ImageSurfaces. This halves the size of the
previous one, but usually ends up using three of them at the current
CACHE_HIGH_WATER setting.
Should result in a smaller memory footprint for sessions with
multiple copies of nearby audio segments (electronic style).
The downside is the larger memory footprint for linear recording
sessions if CACHE_HIGH_WATER > 1 (1 giving a max of two
half - sized cache entries per audio stream).
Use global clip level from prefs. Restore clip colour, \
Merge branch 'waveview_caching_for_upstream' of https://github.com/nmains/ardour into cairocanvas
fix borked logic controlling whether/when to draw clip/outline pixels/lines in waveviews