[gimp] libgimpwidgets: Introduce GIMP_ZOOM_PINCH

[Jehan <jehanp src gnome org>]

commit 51d1ab9bce70eccd0210c8040a7d8787e76474cb
Author: Povilas Kanapickas <povilas radix lt>
Date:   Tue Jan 26 22:14:44 2021 +0200

    libgimpwidgets: Introduce GIMP_ZOOM_PINCH
    
    This zoom mode is handled pretty much the same as GIMP_ZOOM_SMOOTH
    except that we do not apply adjustments for the scaling delta. Pinch
    zooming is ultimately a different physical activity than smooth
    scrolling and users likely expect different sensitivity.

 libgimpwidgets/gimpwidgetsenums.h | 4 +++-
 libgimpwidgets/gimpzoommodel.c    | 8 ++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)
---
diff --git a/libgimpwidgets/gimpwidgetsenums.h b/libgimpwidgets/gimpwidgetsenums.h
index 3544afc6a3..ef6d935607 100644
--- a/libgimpwidgets/gimpwidgetsenums.h
+++ b/libgimpwidgets/gimpwidgetsenums.h
@@ -216,6 +216,7 @@ typedef enum
  * @GIMP_ZOOM_OUT_MAX:  zoom out as far as possible
  * @GIMP_ZOOM_TO:       zoom to a specific zoom factor
  * @GIMP_ZOOM_SMOOTH:   zoom smoothly from a smooth scroll event
+ * @GIMP_ZOOM_PINCH:    zoom smoothly from a touchpad pinch gesture
  *
  * the zoom types for #GimpZoomModel.
  **/
@@ -232,7 +233,8 @@ typedef enum
   GIMP_ZOOM_IN_MAX,    /*< skip >*/
   GIMP_ZOOM_OUT_MAX,   /*< skip >*/
   GIMP_ZOOM_TO,        /*< skip >*/
-  GIMP_ZOOM_SMOOTH     /*< skip >*/
+  GIMP_ZOOM_SMOOTH,    /*< skip >*/
+  GIMP_ZOOM_PINCH,     /*< skip >*/
 } GimpZoomType;
 
 
diff --git a/libgimpwidgets/gimpzoommodel.c b/libgimpwidgets/gimpzoommodel.c
index 6d12aba7c0..e4b772eacc 100644
--- a/libgimpwidgets/gimpzoommodel.c
+++ b/libgimpwidgets/gimpzoommodel.c
@@ -705,6 +705,14 @@ gimp_zoom_model_zoom_step (GimpZoomType zoom_type,
         new_scale = scale;
       break;
 
+    case GIMP_ZOOM_PINCH:
+      if (delta > 0.0)
+        new_scale = scale * (1.0 + delta);
+      else if (delta < 0.0)
+        new_scale = scale / (1.0 + -delta);
+      else
+        new_scale = scale;
+      break;
     }
 
   return CLAMP (new_scale, ZOOM_MIN, ZOOM_MAX);