support for blend modes

[myuon]

Supports blending modes:

• setBlendMode :: (FreeGame m) => Blending -> m ()
• glBlending :: Blending -> IO ()

See my example (examples/blendTest.hs) to make sure that it works fine. Hope this will help.

[fumieval]

It'd be nice to set a blend mode to an entire computation like this:

blendMode NormalAlpha $ translate (V2 140 200) $ bitmap bmp

I'm going to implement blendMode API using your glBlending.