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>

1 files changed, 4 insertions, 1 deletions

diff --git a/src/mesa/state_tracker/st_cb_fbo.c b/src/mesa/state_tracker/st_cb_fbo.c
index 70baa99..daa94df 100644
--- a/src/mesa/state_tracker/st_cb_fbo.c
+++ b/src/mesa/state_tracker/st_cb_fbo.c
@@ -680,7 +680,10 @@ st_DrawBuffers(struct gl_context *ctx, GLsizei count, const GLenum *buffers)
    /* add the renderbuffers on demand */
    for (i = 0; i < fb->_NumColorDrawBuffers; i++) {
       gl_buffer_index idx = fb->_ColorDrawBufferIndexes[i];
-      st_manager_add_color_renderbuffer(st, fb, idx);
+
+      if (idx >= 0) {
+         st_manager_add_color_renderbuffer(st, fb, idx);
+      }
    }
 }