[EPIC] Create the terrain viewer #77

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Same as title

Milestones

Show a height grid in the browser

• Terrain drawn as coloured cells, with a stand-in behind it instead of a backend

Control what is generated from a side panel

• Type a seed, a corner, and a size, press generate, see that region

Move the view around the terrain

• Drag and zoom, close enough to read a single cell

Ask for the terrain you have moved to

• Pan to blank space and generate what is under the view