Regression from commit 0761f816 (123792:
complete annotations on text ranges feature, 2013-12-19), the problem
was that in case a comment had a reply, then both comments produced its
range overlay, and this repeated semi-transparent overlay resulted in
dark color.

Go back to the old behavior of just painting the overlay for the
non-reply comment; for real, partly overlapping comments showing the
exact boundaries of the overlapping overlays still makes sense.

Change-Id: Ic2673a1a41146ff547262ba681b34a63f41aa8ff