With ext-interp, a switchable interpolation filter is coded iff the
motion vector uses fractional pixel movement (ie, true subpixel
movement). With ext-interp and obmc enabled at the same time, the RD
search proceeds as:
1. Do motion search
2. Do interpolation filter search iff subpixel motion, otherwise use
   EIGHTTAP_REGULAR
3. Evaluate obmc=0
4. Evaluete obmc=1 - This involves another motion search

If the motion search in step 4 yields an integer motion vector, while
the search in step 1 did not, then an interp_filter value other than
EIGHTTAP_REGULAR is invalid, and will cause an assertion failure
at output time, or a mismatch if not using --enable-debug.

The fix sets the interp_filter to EIGHTTAP_REGULAR if obmc=1 is picked
with an integer motion vector.

Change-Id: I4685d1ad537f41d833dc9eb64845956b67886cca