Emma Anholt authored Nov 06, 2015 and Adam Jackson committed Jul 18, 2016

It is a modest performance improvement (2.7% on Intel), with the
significant downside that it keeps extra pixmap contents laying around
for 1000 BlockHandlers without the ability for the system to purge
them when under memory pressure, and tiled renderers don't know that
we could avoid reading their current contents when beginning to render
again.  We could use the FB invalidate functions, but they aren't
always available, aren't hooked up well in Mesa, and would eat into
the performance gains of having the cache.

[ajax: rebased to master]

Reviewed-by: Adam Jackson <ajax@redhat.com>