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