Fix examples/webgl_gpgpu_birds_gltf.html blue scene wrong initial size.

[gonnavis]

https://threejs.org/examples/#webgl_gpgpu_birds_gltf

If the blue scene is randomly selected, the initial size is the wrong 0.1 instead of the correct 0.2, and it will suddenly become 0.2 when the other gui parameters are updated.

[gonnavis]

Because the inital size is changed, and the screenshot is just the blue scene, so it'll not pass the E2E testing.

------EDIT------
Made new screenshot.

[mrdoob]

Thanks!