[gimp] app: add/update some comments.

[Jehan Pagès <jehanp src gnome org>]

commit f28cb1c511fda93aad44a49e21838c62b3f5b19f
Author: Jehan <jehan girinstud io>
Date:   Wed Dec 6 23:30:54 2017 +0100

    app: add/update some comments.
    
    Keep multi-threading disable on the cage transform operation. It is now
    fixed but is a lot much slower than when single-threaded, making it
    painful to use on bigger images.
    So let's reenable later if we can improve this. See bug 787663.

 app/operations/gimpoperationcagetransform.c |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)
---
diff --git a/app/operations/gimpoperationcagetransform.c b/app/operations/gimpoperationcagetransform.c
index a94e553..fa17e13 100644
--- a/app/operations/gimpoperationcagetransform.c
+++ b/app/operations/gimpoperationcagetransform.c
@@ -111,8 +111,8 @@ gimp_operation_cage_transform_class_init (GimpOperationCageTransformClass *klass
   operation_class->get_cached_region       = gimp_operation_cage_transform_get_cached_region;
   operation_class->no_cache                = FALSE;
   operation_class->get_bounding_box        = gimp_operation_cage_transform_get_bounding_box;
-  /* XXX Temporarily disable multi-threading on this operation until it
-   * is fixed. See bug 787663.
+  /* XXX Temporarily disable multi-threading on this operation because
+   * it is much faster when single-threaded. See bug 787663.
    */
   operation_class->threaded                = FALSE;
 
@@ -383,6 +383,9 @@ gimp_operation_cage_transform_interpolate_source_coords_recurs (GimpOperationCag
   GeglRectangle rect = { 0, 0, 1, 1 };
   gint          xmin, xmax, ymin, ymax;
 
+  /* Stop recursion if all 3 vertices of the triangle are outside the
+   * ROI (left/right or above/below).
+   */
   if (p1_d.x >= roi->x + roi->width &&
       p2_d.x >= roi->x + roi->width &&
       p3_d.x >= roi->x + roi->width) return;