mod refmvs: backport shrinkage of RefMvsFrame from dav1d 1.5.2#1462

fbossen wants to merge 2 commits intomemorysafety:mainmemorysafety/rav1d:mainfrom

fbossen:bp-1.5.2-refmvfbossen/rav1d:bp-1.5.2-refmvCopy head branch name to clipboard