For some historical reason, coefficients of interpolation filters
in filter.c is defined in int16_t but use 8 bits only.
On the other hand, SIMD like to use int8_t to represent those
coefficients for economical reason.
The conversion from int16_t to int8_t will introduce
"possible loss of data" warning.
We use cast to work around this problem.
Change-Id: I42ef0a2a815a8f93c579f7984c9b957c1aed6d6c