backward-compatible windows with rounded corners

[capr]

Windows with rounded corners can be done trivially with transparent = true option.

But that doesn't work in Linux with non-composite window managers, and it also uses a different code path in Windows (UpdateLayeredWindow instead of WM_PAINT) that doesn't work with RemoteDesktop (see #2) and doesn't allow direct OpenGL/D3D rendering.

The idea with this issue is to have a corner_radius = radius (default 0) window() option that we can use instead of transparent = true:

Windows: CreateRoundRectRgn() ? sigh...
X11: XShape? sigh even more...
OSX: make a clipping path? ignore and let the user make a clipping path?

[capr]

Update: Windows: done (60683a06ef7347d66b2c1d9ebf3deafd2fa78fe0)