Fix translation bugs

The translation needs to be scaled relative to the zoom percentage so
we get sensible movement when dragging. Dragging is also disabled when
the image is smaller than the container widget.

Re-orders the initialisation of some variables to stop a compile
warning on Travis.

app/widget/viewer/viewerdisplay.cpp

@@ -48,7 +48,8 @@ ViewerDisplayWidget::ViewerDisplayWidget(QWidget *parent) :
   gizmos_(nullptr),
   gizmo_click_(false),
   last_loaded_buffer_(nullptr),
-  zoomed_(false)
+  zoomed_(false),
+  zoom_multiplier_(1.0)
 {
 }
 
@@ -69,13 +70,17 @@ void ViewerDisplayWidget::SetMatrixZoom(const QMatrix4x4 &mat)
   update();
 }
 
-void ViewerDisplayWidget::SetZoomFlag(bool flag)
+void ViewerDisplayWidget::SetZoomData(bool flag, int percent)
 {
   zoomed_ = flag;
-  // If not scaled reset translation matrix to identity
+  // If the image is smaller than the conainer widget we disable translation
   if (!flag) {
     QMatrix4x4 mat;
     SetMatrixTranslate(mat);
+    zoom_multiplier_ = 1.0f;
+  }
+  else {
+    zoom_multiplier_ = 1.0 / (static_cast<double>(percent) * 0.01);
   }
 }
 
@@ -174,7 +179,7 @@ void ViewerDisplayWidget::mousePressEvent(QMouseEvent *event)
     SetMatrixTranslate(mat);
   }
 
-  // get current position in preperation for move event
+  // If translation is enabled get current position in preperation for move event
   if (event->button() == Qt::MiddleButton && zoomed_) {
     position_ = event->pos();
     return;
@@ -189,11 +194,12 @@ void ViewerDisplayWidget::mousePressEvent(QMouseEvent *event)
 
 void ViewerDisplayWidget::mouseMoveEvent(QMouseEvent *event)
 {
+  // Only allow translation if the image is larger than the container widget
   if (event->buttons() & Qt::MiddleButton && zoomed_) {
     QPointF delta = event->pos() - position_;
-    // scale delta to widget size
-    delta.setX(delta.x() / width());
-    delta.setY(delta.y() / height());
+    // scale delta to widget size and zoom level
+    delta.setX(zoom_multiplier_ * delta.x() / width());
+    delta.setY(zoom_multiplier_ * delta.y() / height());
 
     QMatrix4x4 mat;
     mat = GetMatrixTranslate();