meteor showers: use gl.canvas for meteor point size

vendor/clouds3d.js

@@ -6538,7 +6538,7 @@ function drawMeteorShower(gl, proj, view, sunAlt) {
     gl.enableVertexAttribArray(meteorUniforms.a1);
     gl.vertexAttribPointer(meteorUniforms.a1, 4, gl.FLOAT, false, 32, 16);
     gl.uniformMatrix4fv(meteorUniforms.mvp, false, mvp);
-    gl.uniform1f(meteorUniforms.sizeScale, canvas.height / 900);
+    gl.uniform1f(meteorUniforms.sizeScale, (gl.canvas && gl.canvas.height ? gl.canvas.height : 900) / 900);
     gl.disable(gl.DEPTH_TEST);
     gl.depthMask(false);
     gl.enable(gl.BLEND);