[gdk-pixbuf] docs: Document existing scaling and transformation fast paths

[Philip Withnall <pwithnall src gnome org>]

commit f4b97747e770650063bf7f6d7c1199ecba5ed54f
Author: Philip Withnall <philip tecnocode co uk>
Date:   Sat Jan 7 19:18:12 2017 +0000

    docs: Document existing scaling and transformation fast paths
    
    There are not many, as far as I can see.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=442452

 gdk-pixbuf/gdk-pixbuf-scale.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/gdk-pixbuf/gdk-pixbuf-scale.c b/gdk-pixbuf/gdk-pixbuf-scale.c
index e6b78c2..202e433 100644
--- a/gdk-pixbuf/gdk-pixbuf-scale.c
+++ b/gdk-pixbuf/gdk-pixbuf-scale.c
@@ -261,6 +261,9 @@ gdk_pixbuf_composite (const GdkPixbuf *src,
  * colors @color1 and @color2 and renders it onto the destination
  * image.
  *
+ * If the source image has no alpha channel, and @overall_alpha is 255, a fast
+ * path is used which omits the alpha blending and just performs the scaling.
+ *
  * See gdk_pixbuf_composite_color_simple() for a simpler variant of this
  * function suitable for many tasks.
  * 
@@ -414,6 +417,8 @@ gdk_pixbuf_composite_color_simple (const GdkPixbuf *src,
  * Rotates a pixbuf by a multiple of 90 degrees, and returns the
  * result in a new pixbuf.
  *
+ * If @angle is 0, a copy of @src is returned, avoiding any rotation.
+ *
  * Returns: (nullable) (transfer full): the new #GdkPixbuf, or %NULL
  * if not enough memory could be allocated for it.
  *