point size attenuation fix

examples/webgl_interactive_points.html

@@ -49,7 +49,7 @@
 
 				vec4 mvPosition = modelViewMatrix * vec4( position, 1.0 );
 
-				gl_PointSize = size * ( 300.0 / length( mvPosition.xyz ) );
+				gl_PointSize = size * ( 300.0 / -mvPosition.z );
 
 				gl_Position = projectionMatrix * mvPosition;