st/mesa: fix renderbuffer validation bug

After we attach a new renderbuffer in this function we need to make
sure Mesa's update_framebuffer() gets called.

Fixes crash in WebGL conformance/textures/texture-attachment-formats.html,
but the test still fails for other reasons.

Fixes http://bugs.freedesktop.org/show_bug.cgi?id=53316

Note: This is a candidate for the 8.0 branch.

Reviewed-by: José Fonseca <jfonseca@vmware.com>

src/mesa/state_tracker/st_cb_fbo.c

@@ -458,6 +458,12 @@ st_render_texture(struct gl_context *ctx,
     * passed to the pipe as a (color/depth) render target.
     */
    st_invalidate_state(ctx, _NEW_BUFFERS);
+
+
+   /* Need to trigger a call to update_framebuffer() since we just
+    * attached a new renderbuffer.
+    */
+   ctx->NewState |= _NEW_BUFFERS;
 }