OGL: Fix a memory leak that would occur every time a game is launched

[lioncash]

The GL interface would always be re-initialized, but the previous instance would never be deleted.

[Mullin]

/Source/Core/VideoBackends/OGL/main.cpp: In member function 'virtual void OGL::VideoBackend::Shutdown()':
/Source/Core/VideoBackends/OGL/main.cpp:227:9: warning: deleting object of polymorphic class type 'cInterfaceBase' which has non-virtual destructor might cause undefined behaviour [-Wdelete-non-virtual-dtor]
delete GLInterface;

[lioncash]

@Mullin should be all good now.

[Tilka]

VideoSoftware supposedly has the same leak.

[Mullin]

LGTM

[lioncash]

@Tilka Should be good now.

[Tilka]

lgtm