Work out a tile's seed from the global seed and its coordinate #37

Description

Create a function that takes a global seed and a tile coordinate and returns the seed to generate that tile with.

The coordinate's only job at this stage is to make one tile's noise differ from another's. Patches do not carry coordinates and neighbouring tiles do not line up with each other yet, so there is nothing here about position or bounds.

Testing:

• Assert the same global seed and coordinate always give the same tile seed
• Assert two different coordinates under the same global seed give different tile seeds
• Assert two different global seeds at the same coordinate give different tile seeds