st/mesa: bind NULL colorbuffers as specified by glDrawBuffers

st/mesa: bind NULL colorbuffers as specified by glDrawBuffers

An example why it is required:

    Let's say there's a fragment shader writing to gl_FragData[0..1].
    The user calls: glDrawBuffers(2, {GL_NONE, GL_COLOR_ATTACHMENT0});

    That means gl_FragData[0] is unused and gl_FragData[1] is written
    to GL_COLOR_ATTACHMENT0.

st/mesa was skipping the GL_NONE draw buffer, therefore gl_FragData[0]
was written to GL_COLOR_ATTACHMENT0, which was wrong.

This commit fixes it, but drivers must also be fixed not to crash when
binding NULL colorbuffers. There is also a new set of piglit tests for this.

The MSAA state also had to be fixed not to crash when reading fb->cbufs[0].

Reviewed-by: Brian Paul <brianp@vmware.com>