Make touchpad gestures optional and allow >100% zooming

This addresses #20750.

* New option darkroom/ui/touchpad_gestures allows to disable
  touchpad based pinch zooming, it is enabled by default
* Touchpad pinch zooming now is on par with old CTRL + scroll
  zooming by allowing to zoom beyond 100%
* Previous CTRL + scroll zooming is now re-enabled for touchpads
  next to the gesture based pinch zooming for a happy coexistance

Author: da-phil

data/darktableconfig.xml.in

@@ -2398,6 +2398,13 @@
     <longdescription><![CDATA[in darktable's darkroom, where you do your edits, all controls are listed on a panel on the right; you can choose whether you want to use the scroll wheel or scroll gesture on your touchpad to scroll the panel, or to change the values of the control under the pointer; this latter modality enables faster editing, but this can be dangerous because you can change the values ​​of the module controls without noticing if you are not used to it\n\n<b>enabled:</b> the mouse wheel scrolls the modules panel and with <i>Ctrl+Alt</i> adjusts a control's value\n\n<b>disabled:</b> the mouse wheel adjusts the control under the pointer and with <i>Ctrl+Alt</i> scrolls the panel.]]></longdescription>
     <welcomescreen pagenum="5" questionnum="1"/>
   </dtconfig>
+  <dtconfig prefs="misc" section="interface">
+    <name>darkroom/ui/touchpad_gestures</name>
+    <type>bool</type>
+    <default>true</default>
+    <shortdescription>enable touchpad gestures in darkroom</shortdescription>
+    <longdescription><![CDATA[use two-finger touchpad gestures in darkroom for panning and pinch-to-zoom. <b>enabled:</b> touchpad pinch gestures zoom the image and two-finger touchpad scrolling pans it; <i>Ctrl+scroll</i> still uses the legacy zoom behavior. <b>disabled:</b> touchpad gestures are ignored and darkroom falls back to the legacy scroll behavior, including <i>Ctrl+scroll</i> for zooming in and out.]]></longdescription>
+  </dtconfig>
   <dtconfig prefs="darkroom" section="general">
     <name>plugins/darkroom/ui/border_size</name>
     <type>int</type>

src/gui/gtk.c

@@ -726,6 +726,21 @@ static gboolean _draw(GtkWidget *da,
 }
 
 static GdkDevice *_touchpad = NULL;
+static gboolean _touchpad_gestures_enabled(void)
+{
+  // If conf_gen.h was built before darktableconfig.xml.in gained this key
+  // (incremental build without cmake reconfigure), dt_confgen_value_exists
+  // returns FALSE and dt_conf_get_bool gets an empty string → FALSE.
+  // Default to enabled in that case so a stale build doesn't silently break gestures.
+  if(!dt_confgen_value_exists("darkroom/ui/touchpad_gestures", DT_DEFAULT))
+  {
+    dt_print(DT_DEBUG_INPUT,
+             "[touchpad] 'darkroom/ui/touchpad_gestures' missing from confgen"
+             " (stale conf_gen.h — run cmake reconfigure), defaulting to enabled");
+    return TRUE;
+  }
+  return dt_conf_get_bool("darkroom/ui/touchpad_gestures");
+}
 
 static gboolean _input_event(GtkWidget *widget,
                              GdkEvent *event,
@@ -744,6 +759,7 @@ static gboolean _input_event(GtkWidget *widget,
   }
 
   if(event->type == GDK_TOUCHPAD_PINCH)
+  if(event->type == GDK_TOUCHPAD_PINCH && _touchpad_gestures_enabled())
   {
     const GdkEventTouchpadPinch *pinch = &event->touchpad_pinch;
     if(dt_view_manager_gesture_pinch(darktable.view_manager, pinch->x, pinch->y,
@@ -764,8 +780,10 @@ static gboolean _scrolled(GtkWidget *widget,
   (void)user_data;
   GdkDevice *device = gdk_event_get_source_device((GdkEvent *)event);
 
-  if(((device && gdk_device_get_source(device) == GDK_SOURCE_TOUCHPAD)
-      || device == _touchpad)
+  if(_touchpad_gestures_enabled()
+     && !dt_modifier_is(event->state, GDK_CONTROL_MASK)
+     && ((device && gdk_device_get_source(device) == GDK_SOURCE_TOUCHPAD)
+         || device == _touchpad)
      && event->direction == GDK_SCROLL_SMOOTH && !event->is_stop)
   {
     gdouble delta_x = 0.0, delta_y = 0.0;

src/views/darkroom.c

@@ -4192,7 +4192,7 @@ gboolean gesture_pinch(dt_view_t *self,
 {
   dt_develop_t *dev = self->data;
   if(!dev) return FALSE;
-  const gboolean constrained = !dt_modifier_is(state, GDK_CONTROL_MASK);
+  (void)state;
   const double pinch_step_ratio = 1.1;
 
   static double pinch_last_scale = 0.0;
@@ -4221,7 +4221,7 @@ gboolean gesture_pinch(dt_view_t *self,
 
   if(zoom_step >= 0)
   {
-    dt_dev_zoom_move(&dev->full, DT_ZOOM_SCROLL, 0.0f, zoom_step, x, y, constrained);
+    dt_dev_zoom_move(&dev->full, DT_ZOOM_SCROLL, 0.0f, zoom_step, x, y, FALSE);
     pinch_last_scale = scale;
   }