New exponent strategy decision

I've implemented and committed a new algorithm for the selection of exponent strategies. The old method was from FFmpeg. It compared the difference between exponents from one block to the next and selected exponent strategy for each block using a threshold. The new method selects between 5 predefined sets of strategies for the whole frame. All 5 use the same number of exponent bits. The selection is done by comparing the original exponents to each set of encoded exponents and choosing the set with the smallest deviation from the original.