Make it possible for forced VP8 SW fallback encoder to set min_pixels_per_frame via GetScalingSettings().
Add a min required resolution (in addition to bitrate) before releasing forced SW fallback.
BUG=webrtc:6634
Review-Url: https://codereview.webrtc.org/3000693003
Cr-Commit-Position: refs/heads/master@{#19390}