Some more cairo tests (and small fixes/improvements)

Hi,

I wanted to enhance tests/cairo.lua some more and this is the result. Unexpectedly, I found things to change in override/cairo.lua, too.

Also, I am not really happy about the current binding for cairo_matrix_multiply(), but I don't have any good suggestions on how to change that and I don't think that function is used much anyway.

If you know about any specific part of the cairo bindings that you'd like to have tests for, just tell me.

Uli