bugfix when ConstrainedIntraPred is turned on
This merge request is related to JVET VVC Trac ticket #125. Encoder and decoder mismaches when ConstrainedIntraPred is turned on. This mismatches can be repeated by using the following configuration: "-c encoder_randomaccess_vtm.cfg -c bqsquare.cfg -q 37 -fs 576 -f 17 --ConstrainedIntraPred=1".
For the neighboring left-side CUs, in current VTM, when ConstrainedIntraPred is turned on, the reference sample filling process is still trying to copy the samples from top to down even though the intra coded CUs under the inter coded CUs. This bug is fixed by reversing the direction of the copying process when above mentioned condition is satisfied.
In addition, this bug doesn't affect the bitstream in CTC condition. This is because that if the left-below neighboring CUs is available, the CUs above that CU is alway available in CTC condition, so the sample copying process can only from top to down.