Shared pixmap destruction is done by unrefing the master pixmap twice: once
for the original reference, and once for the reference implicitly added by
PixmapShareToSlave. Then, unrefing the slave pixmap once.
When I add PRIME double buffering and synchronization, I will need to do
this in multiple places. To avoid duplication of code and comments
explaining it everywhere, factor it out into its own function and use that
in place of where it was before.
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Alex Goins <agoins@nvidia.com>
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