Minor rate control refactoring and experiments.
Some minor refactoring code relating to estimates of bits per MB at a given Q and estimating the allowed Q range. Most of the changes here were included in a previous commit. This commit seeks to separate out the refactoring from more the material changes. Two #define control flags have been added for experimentation. ONE_SHOT_Q_ESTIMATE force the two pass encoder to use its initial Q range estimate for the whole clip even if this results in a miss on the target data rate. In effect this tightens the Q range seen at the expense of rate control accuracy. DISABLE_RC_LONG_TERM_MEM is a related flag that disables the long term memory in the rate control. Local adjustments are still made to try and better hit the rate target on a per frame basis but the impact of rate control misses is not propagated to the remainder of the clip. This means that for example an overshoot early on will not cause frames later in the clip to be starved of bits. Again the result of this relaxation amy be less rate control accuracy especially on short clips. The flags are disabled by default for now. Change-Id: I7482f980146d8ea033b5d50cc689f772e4bd119e
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