Add support for splash screen image transparency

Some controls (notably pan azimuth) have reversed range 1..0.
Support for this was added years ago in 647103c825
but inline controls were not updated.

gtk2_ardour/processor_box.cc

@@ -1036,11 +1036,11 @@ ProcessorEntry::Control::Control (ProcessorEntry& e,std::shared_ptr<AutomationCo
 		_slider.show ();
 
 		const ARDOUR::ParameterDescriptor& desc = c->desc();
-		double const lo        = c->internal_to_interface (desc.lower);
-		double const up        = c->internal_to_interface (desc.upper);
-		double const normal    = c->internal_to_interface (desc.normal);
-		double const smallstep = c->internal_to_interface (desc.lower + desc.smallstep);
-		double const largestep = c->internal_to_interface (desc.lower + desc.largestep);
+		double const lo        = c->internal_to_interface (desc.lower, true);
+		double const up        = c->internal_to_interface (desc.upper, true);
+		double const normal    = c->internal_to_interface (desc.normal, true);
+		double const smallstep = fabs (c->internal_to_interface (desc.lower + desc.smallstep, true) - lo);
+		double const largestep = fabs (c->internal_to_interface (desc.lower + desc.largestep, true) - lo);
 
 		_adjustment.set_lower (lo);
 		_adjustment.set_upper (up);
@@ -1099,7 +1099,7 @@ ProcessorEntry::Control::slider_adjusted ()
 		return;
 	}
 
-	c->set_value ( c->interface_to_internal(_adjustment.get_value ()) , Controllable::NoGroup);
+	c->set_value ( c->interface_to_internal(_adjustment.get_value (), true) , Controllable::NoGroup);
 	set_tooltip ();
 }
 
@@ -1186,7 +1186,7 @@ ProcessorEntry::Control::control_changed ()
 		_button.set_active (c->get_value() > 0.5);
 	} else {
 		// Note: the _slider watches the controllable by itself
-		const double nval = c->internal_to_interface (c->get_value ());
+		const double nval = c->internal_to_interface (c->get_value (), true);
 		if (_adjustment.get_value() != nval) {
 			_adjustment.set_value (nval);
 			set_tooltip ();

gtk2_ardour/splash.cc

@@ -238,6 +238,10 @@ Splash::expose (GdkEventExpose* ev)
 {
 	RefPtr<Gdk::Window> window = darea.get_window();
 
+	/* clear background (for transparent splash images */
+	Glib::RefPtr<Gdk::GC> bg = get_style()->get_bg_gc (STATE_NORMAL);
+	window->draw_rectangle(bg, true, ev->area.x, ev->area.y, ev->area.width, ev->area.height)
+
 	/* note: height & width need to be constrained to the pixbuf size
 	   in case a WM provides us with a screwy allocation
 	*/