VideoCore: Saturate vertex colors before interpolating

During testing, it was discovered that hardware does not interpolate
colors output by the vertex shader as-is. Rather, it drops the sign and
saturates the value to 1.0. This is done before interpolation, such that
(e.g.) interpolating outputs 1.5 and -0.5 is equivalent to as if the
shader had output the values 1.0 and 0.5 instead, with the interpolated
value never crossing 0.0.

This change has been tested against hardware.