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

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