Better view culling against MOAIGrid

[patrickmeehan]

As a user I want better view culling against MOAIGrid so that when grids are drawn in perspective views they won't blow up the quad list with countless tiny tiles.

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The current implementation naively intersects all edges of the frustum with the prop's plane then uses the in-plane bounding rectangle. We should consider scan converting a more snugly fitting polygon to produce a more efficient set of tiles to render. We also need a way to enforce a 'horizon' and/or maximum viewable tiles limit: right now, if grid is set to wrap, we wind up with a huge number of tiles to draw as the camera approaches a side-view of the tilemap.