WindowServer: Support windows with alpha channels. And per-WSWindow opacity.

This patch also adds a Format concept to GraphicsBitmap. For now there are
only two formats: RGB32 and RGBA32. Windows with alpha channel have their
backing stores created in the RGBA32 format.

Use this to make Terminal windows semi-transparent for that comfy rice look.
There is one problem here, in that window compositing overdraw incurs
multiple passes of blending of the same pixels. This leads to a mismatch in
opacity which is obviously not good. I will work on this in a later patch.

The alpha blending is currently straight C++. It should be relatively easy
to optimize this using SSE instructions.

For now I'm just happy with the cute effect. :^)

Applications/Terminal/main.cpp

@@ -89,6 +89,7 @@ int main(int argc, char** argv)
     window->set_should_exit_app_on_close(true);
 
     Terminal terminal(ptm_fd);
+    window->set_has_alpha_channel(true);
     window->set_main_widget(&terminal);
     window->move_to(300, 300);
     window->show();