Added cairo blending modes to support libcairo-1.10.0

I'm not sure if this is relevant, since a dev version of 2 appears to be underway, but I really wanted to be able to use the blending modes I'm spoiled with on libcairo 1.10.0 in C. So I just added them to the cairo.pkg for tolua++ to sort out. Everything works fine here.
Just sharing in case you're interested.
I'm submitting a pull request for master too. It compiles in master, but I haven't tested it. I've switched back to 1.9.0 due to laziness (i.e. don't want to tweak my .conkyrc). :)