Refactor TileEdgeOverlay

[metalgearsloth]

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When trying to screenshot vgroid I noticed a not-insignificant amount of CPU time was spent on TileEdgeOverlay iterating thousands of tiles.

Not entirely sure if this is the best way to slot it into the existing tile rendering as I'm not sure how expensive stuff like GLBuffer ctors are (otherwise I'd just allocate a new one whenever it changed size and slap the edge quads into the same buffer). Alternatively I could make the one buffer and slap edge sprites onto the end which might also be better? PJB would know more.