Bug 1020 - First MSAA FBO frame on a mac osx nvidia card not antialiased

OSX/Nvidia's FBO needs to be cleared before blitting,
otherwise first MSAA frame lacks antialiasing.

GLFBODrawableImpl.initialize(..) can clear
GL.GL_COLOR_BUFFER_BIT and GL.GL_DEPTH_BUFFER_BIT, if used.

FBObject cannot clear the buffer(s) due to it's low-level API,
i.e. it cannot know when the first bind occurs _after_ user
completed FBO setup (attaching buffers).

Hence plain FBObject usage required manual injection
of glClear(..) after setup as demonstrated in GLJPanel.

We may need to elaborate in this case,
i.e. add an FBObject API entry like 'fbo.postInitNotify()'.