Fix third_person_terrain for HTML5 target

It was working this morning in krom and it's not working anymore tonight:
I didn't change and save the template (I checked with github desktop).

I also noted that the browser player is throwing:
`this.heightTextures[0] is undefined` and a black screen.

Black screens happen a lot, even after restarting Blender 2.80.75 on Win10

Start by reproducing third_person_terrain for the HTML5 target in Blender 2.80.75 and inspect the browser player's `this.heightTextures[0] is undefined` error alongside the black screen. Done means the terrain renders in the browser player without that error, while also checking whether the issue depends on the unchanged template or the Win10 environment.

Written by the indexing model from the issue text.