compositing: Implement `cursor` per CSS3-UI § 8.1.1 in the CEF/Mac port.

I'm not sure how we want to handle Linux cursors, and GLFW has no
ability to set cursors (short of disabling it and managing it yourself).
auto merge of #4358 : pcwalton/servo/cursor, r=mbrubeck

I'm not sure how we want to handle Linux cursors, and GLFW has no
ability to set cursors (short of disabling it and managing it yourself).

If you test this in the wild you will probably hit #4357 until that PR lands.