Bug 74853

Some affected tests:
fbo-missing-attachment-blit es2 from -fbo -auto
fbo-missing-attachment-blit es2 to -fbo -auto
fbo-missing-attachment-clear -auto
framebuffer-blit-levels draw depth -fbo -auto
framebuffer-blit-levels draw stencil -fbo -auto
and others...

First bad commit:
commit 9baa45f78b8ca7d66280e36009b6a685055d7cd6
Author: Marek Olšák <marek.olsak@amd.com>
Date:   Wed Jan 8 01:09:15 2014 +0100

    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>


./framebuffer-blit-levels draw depth -fbo -auto
Program received signal SIGSEGV, Segmentation fault.
0xb6ca230f in r300_set_framebuffer_state (pipe=0x80880e8, state=0x84bc9a0)
    at r300_state.c:1020
1020	        r300->screen->cmask_resource == state->cbufs[0]->texture;
(gdb) bt
#0  0xb6ca230f in r300_set_framebuffer_state (pipe=0x80880e8, state=0x84bc9a0)
    at r300_state.c:1020
#1  0xb6affe43 in cso_set_framebuffer (ctx=0x84d16f0, fb=0x84bc9a0)
    at cso_cache/cso_context.c:649
#2  0xb69d0d84 in update_framebuffer_state (st=0x84bbf30)
    at state_tracker/st_atom_framebuffer.c:125
#3  0xb69ce0d3 in st_validate_state (st=0x84bbf30)
    at state_tracker/st_atom.c:202
#4  0xb69d7301 in st_BlitFramebuffer (ctx=0x84787d8, srcX0=0, srcY0=0, 
    srcX1=128, srcY1=128, dstX0=0, dstY0=0, dstX1=128, dstY1=128, mask=256, 
    filter=9728) at state_tracker/st_cb_blit.c:94
#5  0xb6868fb3 in _mesa_BlitFramebuffer (srcX0=0, srcY0=0, srcX1=128, 
    srcY1=128, dstX0=0, dstY0=0, dstX1=128, dstY1=128, mask=256, filter=9728)
    at main/blit.c:509
#6  0xb7eb51ff in stub_glBlitFramebuffer (srcX0=0, srcY0=0, srcX1=128, 
    srcY1=128, dstX0=0, dstY0=0, dstX1=128, dstY1=128, mask=256, filter=9728)
    at /home/Paulie/piglit/tests/util/generated_dispatch.c:1724
#7  0x080498c3 in piglit_display ()
    at /home/Paulie/piglit/tests/spec/arb_framebuffer_object/framebuffer-blit-levels.c:372
#8  0xb7eb0540 in display ()
    at /home/Paulie/piglit/tests/util/piglit-framework-gl/piglit_glut_framework.c:60
#9  0xb7fb88ec in fghcbDisplayWindow () from /lib/libglut.so.3
#10 0xb7fbc507 in fgEnumWindows () from /lib/libglut.so.3
#11 0xb7fb8e6e in glutMainLoopEvent () from /lib/libglut.so.3
#12 0xb7fb9657 in glutMainLoop () from /lib/libglut.so.3
#13 0xb7eb0799 in run_test (gl_fw=0xb7fa1b20 <glut_fw>, argc=3, 
    argv=0xbffff1e4)
    at /home/Paulie/piglit/tests/util/piglit-framework-gl/piglit_glut_framework.c:142
#14 0xb7eae59c in piglit_gl_test_run (argc=3, argv=0xbffff1e4, 
    config=0xbffff108)
    at /home/Paulie/piglit/tests/util/piglit-framework-gl.c:191
#15 0x08048df9 in main (argc=3, argv=0xbffff1e4)
    at /home/Paulie/piglit/tests/spec/arb_framebuffer_object/framebuffer-blit-levels.c:63