Issue #3142 - Filters on-canvas preview doesn't work ...

... immediately after an image precision change

When flushing a projection, make sure it has a buffer, instead of
bailing if it doesn't.  We rely on the image projection's "update"
signal to update the display after certain operations that free the
buffer, which would previously fail to happen, and cause subsequent
flushes to be ignored until the buffer is explicitly accessed.

This fixes commit b07f810273.

app/core/gimpprojection.c

@@ -662,11 +662,11 @@ gimp_projection_flush_whenever (GimpProjection *proj,
                                 gboolean        now,
                                 gboolean        direct)
 {
-  if (! proj->priv->buffer)
-    return;
-
   if (proj->priv->update_region)
     {
+      /* Make sure we have a buffer */
+      gimp_projection_allocate_buffer (proj);
+
       if (now)  /* Synchronous */
         {
           gint n_rects = cairo_region_num_rectangles (proj->priv->update_region);