This change silenced valgrind warnings but broke progs/tests/drawbuffers.
The problem is we don't know the surface's state when we start caching it
(it may or may not be initialized/cleared/etc). So "clearing" it here was
presumptuous. Leaving the code in place (but disabled) for reference and
when using valgrind.
Fixes bug 24401
Brian Paul
768481ed40