Add glfw window_refresh_callback, windows now redraw on linux

Fixes #941.
auto merge of #1414 : dhedlund/servo/issue_941, r=larsbergstrom

Moving another window on top of servo window, or switching screen would result in a blank window until the app was resized.  This was due to a missing glfw callback.

Fixes #941.
style: Reduce allocations in selector matching.

This is a 27% improvement in CSS selector matching performance.