Update clutter to 1.2.6.

Clutter 1.2.6 (19/04/2010)
===============================================================================

  * List of changes since Clutter 1.2.4

    > Check if the TexturePixmap actor have an allocation before
      asking for its allocation box, to avoid a costly re-layout.

    > Handle spurious GLX_BufferSwapComplete events more gracefully
      than an assertion failure.

    > Fix a memory leak in the atlas.

    > Use the right constant when calling GetClassLongPtr() in the
      win32 event handling code; this fixes a compilation issue on
      win64.

    > Check the size of the Pango layouts in the cache to avoid
      blowing cache slots.

    > Remove the need for using a function introduced by GLib 2.18,
      to keep our current requirements.

    > Documentation fixes in Clutter and COGL

Many thanks to:

  Fridrich Strba
  Neil Roberts
  Owen W. Taylor
  Rob Bradford
  Robert Bragg

(obache)