export btHeightfieldTerrainShape to PyBullet. Note that tinyrenderer doesn't support rendering it (it would be too slow on CPU)

fix rare getKeyboardEvents threading issue
change texture color to default plane.urdf blue

Extras/Serialize/BulletWorldImporter/btWorldImporter.h

@@ -197,6 +197,12 @@ public:
 
 	virtual btTriangleIndexVertexArray* createMeshInterface(btStridingMeshInterfaceData& meshData);
 
+	virtual class btHeightfieldTerrainShape* createHeightfieldShape(int heightStickWidth, int heightStickLength,
+		const void* heightfieldData, btScalar heightScale,
+		btScalar minHeight, btScalar maxHeight,
+		int upAxis, int heightDataType,
+		bool flipQuadEdges);
+
 	///acceleration and connectivity structures
 	virtual btOptimizedBvh* createOptimizedBvh();
 	virtual btTriangleInfoMap* createTriangleInfoMap();