[Tilecache] Patch for v1.8.1 Watermarking and Transparent Composites

[Andrew Hughes]

Hey All,

To cut a long story short I had missed a test case on the watermarking. 
I had tested transparent watermark image over non transparent tile. When 
both are transparent it hit some code which contained a reference to a 
deprecated variable.

I've created a ticket http://dev.tilecache.org/ticket/9 and I've 
uploaded the patch too (very minor change).


Over and Out
--AH