For ext-intra directional modes, filter intra edge samples
dependent on block size and prediction angle.

Congiguration:
./configure --enable-experimental --enable-ext-intra
  --enable-internal-stats --disable-cdef --disable-delta-q

Coding gain (1 frame Intra):
cif:    -0.03%
midres: -0.01
720p:   -0.65
1080p:  -0.83
4k:     -1.19

Change-Id: I372bfc7614f6a07fe1d8e7b61bd0b6660f9fb4a1