Initialize member variables in MythRenderOpenGL ctor

The refactor of the OpenGL code appears to result
in the ResetVars and ResetProcs functions being
called from the ctors of MythRenderOpenGL[12], but
invoke them explicitly to keep Coverity happy.

Fixes Coverity 700926 and 700927

Fixes #11660.

Signed-off-by: Paul Harrison <pharrison@mythtv.org>