[REVIEW-3-5] Clear sprites to white fdo#45219.

Hi there,

  http://cgit.freedesktop.org/libreoffice/core/commit/?id=b9266113514657be1cf9352e1457a5c62876784c

uses transparent-white to clear the update area in an animated layer
during slideshow. This is an especially ugly fix, since it's
exposing the vclcanvas 1bpp-no-true-compositing shortcoming to
client code. Alas, there's no real other fix, beyond re-doing lots
of vclcanvas (or, rather more preferred, retiring it completely).

Note that for real alpha, this shouldn't make a difference -
(1,1,1,0) is still (0,0,0,0) premultiplied...
 
...we need to double-check on windows though. ;)

Cheers,

-- Thorsten

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