app: add gimp_display_shell_show_image()

... which controls whether or not the image is rendered by the
shell.  We'll use this to hide the image while showing its
transform preview in the next commits.

app/display/gimpdisplayshell-callbacks.c

@@ -509,17 +509,20 @@ gimp_display_shell_canvas_draw_image (GimpDisplayShell *shell,
       gimp_display_shell_draw_checkerboard (shell, cr);
       cairo_restore (cr);
 
-      cairo_set_matrix (cr, &matrix);
+      if (shell->show_image)
-
-      for (i = 0; i < clip_rectangles->num_rectangles; i++)
         {
-          cairo_rectangle_t rect = clip_rectangles->rectangles[i];
+          cairo_set_matrix (cr, &matrix);
 
-          gimp_display_shell_draw_image (shell, cr,
+          for (i = 0; i < clip_rectangles->num_rectangles; i++)
-                                         floor (rect.x),
+            {
-                                         floor (rect.y),
+              cairo_rectangle_t rect = clip_rectangles->rectangles[i];
-                                         ceil (rect.width),
+
-                                         ceil (rect.height));
+              gimp_display_shell_draw_image (shell, cr,
+                                             floor (rect.x),
+                                             floor (rect.y),
+                                             ceil (rect.width),
+                                             ceil (rect.height));
+            }
         }
     }
 

app/display/gimpdisplayshell.c

@@ -314,6 +314,8 @@ gimp_display_shell_init (GimpDisplayShell *shell)
   shell->scale_x     = 1.0;
   shell->scale_y     = 1.0;
 
+  shell->show_image  = TRUE;
+
   gimp_display_shell_items_init (shell);
 
   shell->icon_size       = 128;
@@ -1725,6 +1727,20 @@ gimp_display_shell_mask_bounds (GimpDisplayShell *shell,
   return (*width > 0) && (*height > 0);
 }
 
+void
+gimp_display_shell_set_show_image (GimpDisplayShell *shell,
+                                   gboolean          show_image)
+{
+  g_return_if_fail (GIMP_IS_DISPLAY_SHELL (shell));
+
+  if (show_image != shell->show_image)
+    {
+      shell->show_image = show_image;
+
+      gimp_display_shell_expose_full (shell);
+    }
+}
+
 void
 gimp_display_shell_flush (GimpDisplayShell *shell)
 {

app/display/gimpdisplayshell.h

@@ -90,6 +90,8 @@ struct _GimpDisplayShell
 
   gboolean           proximity;        /*  is a device in proximity           */
 
+  gboolean           show_image;       /*  whether to show the image          */
+
   Selection         *selection;        /*  Selection (marching ants)          */
 
   GList             *children;
@@ -302,6 +304,10 @@ gboolean          gimp_display_shell_mask_bounds   (GimpDisplayShell   *shell,
                                                     gint               *width,
                                                     gint               *height);
 
+void              gimp_display_shell_set_show_image
+                                                   (GimpDisplayShell   *shell,
+                                                    gboolean            show_image);
+
 void              gimp_display_shell_flush         (GimpDisplayShell   *shell);
 
 void              gimp_display_shell_pause         (GimpDisplayShell   *shell);