Use ref_frame_map vice active_ref_idx on the encoder

This patch makes the encoder's use of ref_frame_map and active_ref_idx
consistent with the decoder. ref_frame_map[] maps a reference buffer
index to its actual location in the yv12_fb array, since many
references may share an underlying buffer. active_ref_idx[] mirrors
cpi->{lst,gld,alt}_fb_idx, holding the active references in each
slot.

This also fixes a bug in setup_buffer_inter() where the incorrect
reference was used to populate the scaling factors.

Change-Id: Id3728f6d77cffcd27c248903bf51f9c3e594287e