Fix short term reference picture list parsing. Before this change, `deltaPocS0`
was derived by adding one to the value of the syntax element
`delta_poc_s0_minus1`, but (maybe surprising) the specification actually says
that `DeltaPocS0[stRpsIdx][i]` should be assigned the negation
`-(delta_poc_s0_minus1[i] + 1)` on the first iteration, then that value added
to the previous value on previous iterations. See equations (7-67) to (7-70) in
the 2021-08 version of the H.265/HEVC specification.
Also read the number of long term reference pictures once rather than on every
loop iteration (subsection 7.3.2.2.1).
PiperOrigin-RevId: 551852999
(cherry picked from commit 2fe67266a1d13c69e09c37b0aadf6015bb37e7b7)
