Add a comment about the interactions of negative pitches, overlapping

blits and the different ways of specifying a blit on this hardware.

src/mesa/drivers/dri/i965/intel_blit.c

@@ -288,6 +288,14 @@ void intelEmitCopyBlit( struct intel_context *intel,
 
    /* Initial y values don't seem to work with negative pitches.  If
     * we adjust the offsets manually (below), it seems to work fine.
+    *
+    * However, on broadwater at least, only the top version works
+    * correctly with overlapping blits.  Luckily we don't need
+    * negative pitches and overlapping blits at the same time, as far
+    * as I know anyhow.
+    *
+    * Further, the current i965 driver never requires negative
+    * pitches, so just use the old-style blits for now.
     */
    if (1) {
       BEGIN_BATCH(8, INTEL_BATCH_NO_CLIPRECTS);