Make dixDestroyPixmap() check for NULL pointer, so callers don't need to
do it anymore. Returning TRUE on NULL pointer - but most callers won't
even look at the retval anyways.
Together with subsequent commits, which will make use of that function,
instead of calling raw ScreenRec->DestroyPixmap vectors, this gives us some
more freedom for architectural changes, eg. get rid of the extremely
complicated and fragile wrapping chains.
Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1709>
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