app: add gimp_gegl_clear()

... which clears the alpha component of a given buffer region,
i.e., it makes the region transparent, while preserving color
information.  This corresponds to the "edit-clear" action.

app/gegl/gimp-gegl-loops.cc

@@ -93,6 +93,58 @@ gimp_gegl_buffer_copy (GeglBuffer          *src_buffer,
     }
 }
 
+void
+gimp_gegl_clear (GeglBuffer          *buffer,
+                 const GeglRectangle *rect)
+{
+  const Babl *format;
+  gint        bpp;
+  gint        n_components;
+  gint        bpc;
+  gint        alpha_offset;
+
+  g_return_if_fail (GEGL_IS_BUFFER (buffer));
+
+  if (! rect)
+    rect = gegl_buffer_get_extent (buffer);
+
+  format = gegl_buffer_get_format (buffer);
+
+  if (! babl_format_has_alpha (format))
+    return;
+
+  bpp          = babl_format_get_bytes_per_pixel (format);
+  n_components = babl_format_get_n_components (format);
+  bpc          = bpp / n_components;
+  alpha_offset = (n_components - 1) * bpc;
+
+  gegl_parallel_distribute_area (
+    rect, PIXELS_PER_THREAD,
+    [=] (const GeglRectangle *area)
+    {
+      GeglBufferIterator *iter;
+
+      iter = gegl_buffer_iterator_new (buffer, rect, 0, format,
+                                       GEGL_ACCESS_READWRITE, GEGL_ABYSS_NONE,
+                                       1);
+
+      while (gegl_buffer_iterator_next (iter))
+        {
+          guint8 *data = (guint8 *) iter->items[0].data;
+          gint    i;
+
+          data += alpha_offset;
+
+          for (i = 0; i < iter->length; i++)
+            {
+              memset (data, 0, bpc);
+
+              data += bpp;
+            }
+        }
+    });
+}
+
 void
 gimp_gegl_convolve (GeglBuffer          *src_buffer,
                     const GeglRectangle *src_rect,

app/gegl/gimp-gegl-loops.h

@@ -28,6 +28,9 @@ void   gimp_gegl_buffer_copy           (GeglBuffer               *src_buffer,
                                         GeglBuffer               *dest_buffer,
                                         const GeglRectangle      *dest_rect);
 
+void   gimp_gegl_clear                 (GeglBuffer               *buffer,
+                                        const GeglRectangle      *rect);
+
 /*  this is a pretty stupid port of concolve_region() that only works
  *  on a linear source buffer
  */