Replace getWindowInfoFromWindow function with user pointer

[bryphe]

Right now, we use the getWindowInfoFromWindow function to take a raw GLFW pointer to a Window, and get our augmented WindowInfo struct from it.

However, turns out there is a better way - there's a glfwSetWindowUserPointer and glfwGetWindowUserPointer to store app-specific data. We should use this instead, and store a pointer to the WindowInfo struct.

We can then use this to replace the getWindowInfoFromWindow, and remove the sActiveWindows concept entirely.