The encoder signals the interp filter type in the frame header if all
blocks use the same filter (see bitstream.c:fix_interp_filter). This
decision is made based on the counts, but with ext-interp, the counts
are actually only incremented for blocks that fail vp10_is_interp_needed
(see for example encodeframe.c:update_state), otherwise a default value
is used (EIGHTTAP_REGULAR). The decoder however first checks if the
interp filter is signaled at the frame level, and uses that filter type
for all blocks, even if the default value should have been used.

This patch makes the decoder first check with vp10_is_interp_needed
to see if the default value should be used and then checks the frame
level signaling, which reconciles the difference between encoder and
decoder.

Change-Id: I87857ade42dea06b0d5ec2a029e9219268334dbb