radeon-rewrite: Crash with incompletely setup renderbuffer

[Nicolai H. <nha...@gm...>]

Certain tests (e.g. piglit's crossbar) crash because they do (possibly 
somewhat weird things) with glReadBuffer().

What happens is this:
- Context gets set up, made current
- Stuff gets rendered
- glReadBuffer() is called to change the read buffer
- glReadPixels() crashes because now ctx->ReadBuffer->_ColorReadBuffer has no 
corresponding buffer object

The issue is that the front buffer's buffer object is never opened. Buffer 
objects for renderbuffers are opened in radeon_update_renderbuffers(), but the 
front buffer buffer object is only opened if we are in single-buffer mode or 
rendering to the front buffer. This just seems incorrect, since clearly *read* 
and *draw* buffer may be different.

On the other hand, there was probably a reason to put this check there in the 
first place, and I would like to understand that reason before blindly 
changing the code. Can somebody enlighten me?

In the meantime, the attached patch fixes the crash for me.

cu,
Nicolai