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. :^)

SharedGraphics/Color.cpp

@@ -23,5 +23,5 @@ Color::Color(NamedColor named)
     default: ASSERT_NOT_REACHED(); break;
     }
 
-    m_value = (rgb.r << 16) | (rgb.g << 8) | rgb.b;
+    m_value = 0xff000000 | (rgb.r << 16) | (rgb.g << 8) | rgb.b;
 }