updates clamp in rec709, rec2020, and rgbMonitor ODTs to only clamp at 0, for consistency with p3 odts
changes clamp function to clamp to range 0-1

  * Sets the clamping of device code values prior to display encoding function to clamp at 0 and 1. Previously, it only clamped at 0.
  * Changes the scaling at end of transforms to divide by the pre-defined constant, MAX_CV, that is defined in the header parameters, rather than recomputing the maximum code value. Should help with readability.