request: Support for borderless windows

For http://github.com/servo/servo we draw the entire window ourselves with OpenGL, so that we can perform global optimizations to reduce GPU state changes and so forth. This means that we need the ability to create a window without a title bar. It also, somewhat surprisingly, means that we need a transparent window, because on Mac OS X windows have rounded corners and the client is responsible for drawing this. Generally, windowing systems have to be explicitly informed that windows are to be opaque in order to enable occlusion culling.

There are a number of ways we could do this: we could have a `uiNewBorderlessWindow(const char *title, int width, int height, int hasMenubar, int transparent)` call, or we could create a `uiWindowOptions` type of structure and refactor the `uiNewWindow` call to use that. Any thoughts?