This experiment modifies lv_map to make use of multi symbol.
Replace the nz_map and coeff_base binary CDF with a new multi-symbol
CDF of size 4. The new base_cdf indicates for each coeff if the level
is 0, 1, 2 or >2. Two new special contexts are added to be used for the
last coefficient (the EOB coeff). For the EOB coefficient we already know
that it is non-zero. We use one context for DC EOB and one for AC EOB
(this can potentially be refined more).
The new symbol is read/written by special bitreader/bitwriter functions.
Those functions reduce the probability precision from 15bit to 9bit before
the invocation of the arithmetic coding engine.
The adapted symbol count is significantly reduced by this experiment.
E.g. for the I-frame of ducks_take_off at cq=12, the number of adapted symbols
is reduced from 6.7M to 4.3M.
Change-Id: Ifc3927d81ad044fb9b0733f1e54d713cb71a1572