Single-plane variant of CfL predictor
This addresses issue #36 (closed). A new function is added to Dav1dIntraPredDSPContext
for predicting a single plane.
Alpha values are checked for zeroes in recon_b_intra to decide whether to:
- predict both planes from AC and first rows of DC;
- predict the zero-alpha plane directly by DC and the non-zero plane by AC and first row of DC.