Limit min Q for normal frames.

Limit min Q for normal frames.

This patch limits the active min Q for normal frames based on the previous
KF/GF/ARF. In a few cases, especially at the end of a clip where there
has been systemic underspend, (as is often the case with slide shows),
this prevents the encoder rapidly dropping Q on normal frames (just to
try and use up bits), such that they end up with a lower Q than the key
frame / GF / ARF off which they key.

Change-Id: Ic8def5c0d1e37ca2202e007ec1d13e501c0a91dd