rrd.php: added helper 'colbright' to adjust color brightness

[simonmeggle]

Inspired by Stackoverflow.

Graphs look pretty nice if AREA is repainted with a darker LINE1.
To define a brighter color than the one in $color_runtime_line, just write

$color_runtime_area = rrd::colbright($color_runtime_line,35);

Negative numbers will darken the color:

$color_runtime_line = rrd::colbright($color_runtime_area,-35);