Fix meter colour range to again have a hard knee at 0dB and twiddle colours so >0dB...

Fix meter colour range to again have a hard knee at 0dB and twiddle colours so >0dB levels are red and stand out.

I am not sure if this is the original intent here, but it seems the most informative/useful/standard/etc. Unfortunately the colour names in the configuration (min, mid, max, clip) are a bit confusing (starting to clip = "mid"?) but changing them would break things. Oh well.

git-svn-id: svn://localhost/ardour2/branches/3.0@10042 d708f5d6-7413-0410-9779-e7cbd77b26cf