add raycast accelerator for btHeightfieldTerrainShape, thanks to Marc Zylann, see https://github.com/bulletphysics/bullet3/pull/2062

it can be disabled by setting the flag cb.m_flags |= btTriangleRaycastCallback::kF_DisableHeightfieldAccelerator;
acceleration is disabled for z axis up.
add btHeightfieldTerrainShape example to example browser

examples/CommonInterfaces/CommonRigidBodyBase.h

@@ -439,12 +439,15 @@ struct CommonRigidBodyBase : public CommonExampleInterface
 
 	virtual void renderScene()
 	{
+		if (m_dynamicsWorld)
 		{
-			m_guiHelper->syncPhysicsToGraphics(m_dynamicsWorld);
-		}
+			{
+				m_guiHelper->syncPhysicsToGraphics(m_dynamicsWorld);
+			}
 
-		{
-			m_guiHelper->render(m_dynamicsWorld);
+			{
+				m_guiHelper->render(m_dynamicsWorld);
+			}
 		}
 	}
 };