Switch to use the normative (convolve8) filter for source scaling,
only for 1/2x1/2 scaling for now. This is faster and has better
quality than either the vpx_scale_frame or the nonnormative scaler.

Remove the vp9_scale_if_required_fast, which is now not used.

Change-Id: I2f7d73950589d19baafb1fa650eac987d531bcc8