If a user added a device on the commandline, it's likely that they want
this device to be the default.

This allows Purr Data to work nicely inside a Flatpak sandbox, as we
can use `-alsaadd pulse` to enable audio output to PulseAudio by
default, with direct hardware access still an option.

This could lead to an assert failure inside ALSA if the output device
wasn't open and we tried to configure it anyway.

As the comment above the alsa_checkiosync() function suggests, there's
no sync to be done if we only have an input or an output.

The (input) and (output) markers were missing in some cases, and
incorrect in others.

- Disable touch-scrolling in situations where it interferes with other
  actions, like operating a slider, or dragging objects or the selection
  rectangle in edit mode.

- Truncate fractional touch coordinates which were causing glitches in the
  FUDI interface.

- Make dragging of iemgui labels and scalar draggables work (kind of).

TODO: Dragging (specifically in edit mode) can be a bit tricky in cases where
it's difficult to hit the right spot to initiate the drag action. This
*might* be solvable by making the algorithm identifying these spots a bit more
lenient, and accept touches which are a bit off.

Also, despite my best efforts, dragging iemgui label handles in edit mode
still needs quite some faffing about when using touch: After clicking the
iemgui object to make the handle appear, you first need to drag the handle a
little (horizontally, vertically, or diagonally) to make the corresponding
drag cursor appear, then release and touch the drag cursor *again* to drag the
handle to its final destination (and the label along with it). It goes without
saying that this seems overly complicated, but there doesn't seem to be an
easy way to work around this in the GUI, as the whole process goes back and
forth between GUI and engine in several steps in the current implementation.

Fixes the regression where the viewport is way too large after an optimal zoom operation (cf. #660).