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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 ddb0f8660489c06ce65d9f4b0256a8b60201920d)