Account for leaf texture in colour

[owengage]

We extract the colour of leaves from the foilage.png. Technically leaves still have a texture. This texture is grey, and combined with the foilage colour to get the final texture.

To be colour accurate we should get the average grey of the texture and blend that with the foilage texture.

This seems particularly glaringly wrong for mangrove leaves, but may be due to having the wrong climate values.