For some reason, grabbing the magic keyboard focus makes scroll stop working
regardless of what MRV::canvas_group_event() returns. I can't figure out any
reason to grab the keyboard in this case anyway, so I just removed it.
Also simlify MRV event handling code in general.
I attempted to preserve the "don't draw unless different" by ditching rounding for more
precise display_span, but that didn't work. An alternative solution would be
to draw on adjustment change if there's text, since then we need to redraw
regardless of slider position, but it seemed weird even just with respect to
the slider, so I opted for this, which really definitely redraws when the
adjustment changes, period.
If this proves to be a performance issue we'll have to figure that out.
Hopefully-desired behaviour is that controls created in the GUI are linear, so
clicking in stuff works like other automation, but controls that originated
from recording are set to discrete so Ardour plays back the input exactly,
instead of doing crazy things like linear interpolation of already high-rate
user input, hold pedals, and so on.
Hopefully that remains the desired behaviour, because we're basically screwed
for ever making any control discrete by default, since we only save the mode to
XML at all if it's not the default, which is currently linear.
Shoot for roughly 30 steps for all controls.
Always keep sensible step information in ParameterDescriptor and just convert
for the UI.
This is a little weird, but it's less weird than it was before, and works.
backends - once instantiated - keep a reference to the engine.
when audioengine is destroyed, the backends must be deinstantiated.
This fixes various unit-test cases.
