tmxrasterizer as a git diff tool: override search path, and allow explicitly writing to stdout

Two little features would make it much easier to plug tmxrasterizer into git as a map differ:

1. An argument for overriding the initial search path. The problem is, when git diffs two versions of the same file, it has to put at least one of them in a temporary file, and now it doesn't know where to find any of its tilesets. I know the original full path, but I have no way to provide it.

2. Writing the image to stdout. This isn't strictly necessary since tempfiles are always an option, but it did surprise me a little that giving - as the outfile path literally created a file named - :)

With both, I believe you could diff a map against its committed version with a oneliner (in zsh, anyway):

compare -metric PHASH =(tmxrasterizer -s 0.25 -p $(dirname $1) $2 -) =(tmxrasterizer -s 0.25 -p $(dirname $1) $5 -) png:- | display -title "$1" -