if a source returns a tile that contains only a single color, add a configurable behavior of symlinking to an existing tile of the same color.

working patch to implement blank tile detection.

I'm not applying this until further investigation: - limit on the number of hard links that can point to a file - if using hard links isn't possible due to the previous limitation, what's the impact in terms of performance and storage of using soft links (knowing that a blank png can be compressed down to a file 132 bytes long)

This issue was closed by revision r34.