Git-Master Merge Request #16: GTK3: prevent V4L2 control feedback loop during GUI refresh (merged)

Integer controls use a GtkScale and GtkSpinButton backed by the same
GtkAdjustment.

During gui_gtk3_update_controls_state(), slider_changed() is blocked
before gtk_range_set_value(), but spin_changed() is still enabled at
that point. Since both widgets share the GtkAdjustment, refreshing the
scale can emit a value change through the spin button and write the
read-back value back to the V4L2 device.

I reproduced this with a Logitech GROUP camera on Fedora 44.
With both hardware autofocus and guvcview software autofocus disabled,
setting Focus Absolute could result in a slightly different read-back
value (for example SET 157 -> GET 156). The unpatched guvcview would
then progressively write back new values on successive refreshes.

With guvcview closed, the V4L2 value remained stable.

I applied this fix to Fedora's guvcview 2.1.0 package, rebuilt the RPM,
and tested it with the same Logitech GROUP camera. The feedback loop
was eliminated: after the camera adjusted the requested value, the
read-back value remained stable instead of continuing to decrease.

This patch ports the same fix to current upstream master.

The Fedora 2.1.0 version of the fix was hardware-tested successfully.
I no longer have access to the camera, so this upstream-master port has
only been build-tested.