Interval parameter is ignored in multiband case

[dorukozturk]

Changing the interval parameter is not doing anything on the RGB case. We might want to add that to vrt configuration if that is possible. I can investigate this.

This produces the same tiles:

M.add_layer(rd[1, 2, 3], interval=(0,1), gamma=2, opacity=1.0)

as this:

M.add_layer(rd[1, 2, 3], interval=(0,999999), gamma=2, opacity=1.0)

[kotfic]

Interval is actually used for calculating colormaps min/max (especially for data that is too large to calculate min/max). It would be nice if we could get this to work with RGB also, but it is not a high priority.