added option to merge hevc in a multi-row grid when possible
added option to merge hevc in a multi-row grid when possible

rbouqueau/gpac,gpac/gpac,rbouqueau/gpac,gpac/gpac,rbouqueau/gpac,rbouqueau/gpac,RodolpheFouquet/gpac,rbouqueau/gpac,RodolpheFouquet/gpac,gpac/gpac,rbouqueau/gpac,gpac/gpac,gpac/gpac,gpac/gpac,gpac/gpac,RodolpheFouquet/gpac,RodolpheFouquet/gpac,rbouqueau/gpac,gpac/gpac,rbouqueau/gpac,RodolpheFouquet/gpac,RodolpheFouquet/gpac