Fix #1580: L1 RPL filling for incomplete GOP

this fixes #1580. There will be an slight impact on the CTC results, as it slightly modifies L1 reference list for the last incomplete GOP. Basically 1 ref is added to the L1 list for some Tid 4 frames. For example, the bdrate difference for class A and B classes subsampled by 2 is not visible in the xls sheet (not enough precision): Tango2 0.000% 0.001% 0.000% FoodMarket4 0.000% 0.000% -0.002% CatRobot1 -0.001% 0.003% 0.004%

MarketPlace 0.000% -0.002% -0.001% RitualDance -0.004% 0.005% 0.012% Cactus -0.002% -0.001% -0.001% BasketballDrive 0.003% -0.009% -0.001% BQTerrace 0.003% 0.000% 0.000%