zink: fix memleaks

This fixes a bunch of minor memleaks in Zink, mostly related to context or screen destruction.

With the exception of the vertex element state, these aren't the kind of resources that keeps accumulating over the life-time of an application, so they should be fairly benign. But better to fix them than not, no?

With these, valgrind --leak-check=full no longer complains about any leaks in Zink for glxinfo. There's still a few leaks in the DRI / GLX code-paths, but these seems unrelated to Zink.