GPGPU Protoplanets example: fix pixel trails (#21510)

examples/webgl_gpgpu_protoplanet.html

@@ -221,6 +221,8 @@
 
 			void main() {
 
+				if ( vColor.y == 0.0 ) discard;
+
 				float f = length( gl_PointCoord - vec2( 0.5, 0.5 ) );
 				if ( f > 0.5 ) {
 					discard;