Adding vp9_get_filter_kernel() function.

Moving INTERPOLATIONFILTERTYPE enum and subpix_fn_table struct to
vp9_filter.h. Adding convenient typedef for subpel kernels.

Function vp9_setup_interp_filters() besides setting xd->subpix.filter_x &
xd->subpix.filter_y has a side effect of also setting scale factors. This
is not required inside decode_modes_b() because scale factors have been
already set by set_ref() calls. That's why replacing
vp9_setup_interp_filters() call with newly created vp9_get_filter_kernel()
call. The behavior of vp9_setup_interp_filters() is unchanged (it
is used from the encoder).

Change-Id: I3f36d3f7cd8d15195a6e2fafd1777cdaf9ecb847