Call glXMakeCurrent from SkNativeSharedGLContext destructor

Servo will destroy DrawTargets from multiple threads, and OpenGL calls will
crash unless we call glXMakeCurrent first.

(I've confirmed that this is still needed on the CPU rendering branch.)
Fix memory leak from bad refcounting

We created the GrContext, with initial refcount = 1, so we can retain it in
SkNativeSharedGLContext without calling AddRef.
Merge pull request #26 from kmcallister/refcount

Fix crash and memory leak