Eliminate memory leak in MythRenderOpenGL

In commit 99e0e577d7f5aa789fbeabffae8b989a61a72288
a memory leak was introduced.  This patch eliminates
the use of the pointer where the lock member object
is lost and replaces it by appropriate creation in
the initializer.  This is a "minimal" fix to the issue.

Signed-off-by: Gary Buhrmaster <gary.buhrmaster@gmail.com>