Without tempmv-signaling configured, using the previous frame's MVs
requires that the last frame was a show frame. With tempmv-signaling
configured, cm->show_last_frame is not checked when calculating
use_prev_frame_mvs. This patch adds that check and resolves mismatches
seen with random resizing and random superres.

Includes a couple fixes too - cm's last_width, last_height, and
last_show_frame were updated under different conditions. Now they're all
updated at the same time.

Change-Id: Ibdfb196cb6e9d002fd57cb4df10a899b60faac00