Add a btIDEbugDraw::clearLines, helps multi-threaded rendering of lines (while updating those lines in a dynamics world in a different thread)

Expose COV_ENABLE_VR_RENDER_CONTROLLERS, to enable/disable rendering of controllers (and some frames) in VR
Expose COV_ENABLE_RENDERING to enable/disable rendering.
Fix some multi-threading issues (potential crashes), related to debug drawing/rendering in one thread, while changing the dynamics world/removing/resetSimulation in a different thread.

src/BulletCollision/CollisionDispatch/btCollisionWorld.cpp

@@ -1514,6 +1514,8 @@ void	btCollisionWorld::debugDrawWorld()
 {
 	if (getDebugDrawer())
 	{
+		getDebugDrawer()->clearLines();
+
 		btIDebugDraw::DefaultColors defaultColors = getDebugDrawer()->getDefaultColors();
 
 		if ( getDebugDrawer()->getDebugMode() & btIDebugDraw::DBG_DrawContactPoints)

src/BulletDynamics/Featherstone/btMultiBodyDynamicsWorld.cpp

@@ -798,6 +798,8 @@ void	btMultiBodyDynamicsWorld::debugDrawWorld()
 {
 	BT_PROFILE("btMultiBodyDynamicsWorld debugDrawWorld");
 
+	btDiscreteDynamicsWorld::debugDrawWorld();
+
 	bool drawConstraints = false;
 	if (getDebugDrawer())
 	{
@@ -867,7 +869,7 @@ void	btMultiBodyDynamicsWorld::debugDrawWorld()
 		}
 	}
 
-	btDiscreteDynamicsWorld::debugDrawWorld();
+	
 }
 
 

src/LinearMath/btIDebugDraw.h

@@ -469,6 +469,10 @@ class	btIDebugDraw
 		drawLine(transform*pt2,transform*pt3,color);
 	}
 
+	virtual void clearLines()
+	{
+	}
+
 	virtual void flushLines()
 	{
 	}