Seperate FILTER_INTRA from EXT_INTRA experiment

Prepare for the av1/nextgenv2 merge.

Coding gain (%):

               lowres     midres
ext-intra       0.69       0.97
filter-intra    0.67       0.83
both            1.05       1.48

Change-Id: Ia24d6fafb3e484c4f92192e0b7eee5e39f4f4ee6