JVET-N0242: non-linear ALF (clipping)

This adds Non-linear ALF filters as in JVET-N0242 (Test-2).

Compared to CE code:

• other tests were removed
• code for JVET_M0132_APS and for JVET_M0277_FIX_PCM_DISABLEFILTER was merged
• alignment on draft text was done (derivation of the clipping values from the internal bitdepth)
• added encoder options UseNonLinearAlfLuma and UseNonLinearAlfChroma to enable/disable non-linear filters optimization