Update only the frame UVs for one billboard sprite.
The sprite keeps its explicit sizeWorld/saved size. Pixel frame dimensions
do not imply a world-space resize; call updateBillboardSpriteIndex with
both frame and sizeWorld when that is desired. Existing flip state is
preserved for non-degenerate UV ranges.
Update only the frame UVs for one billboard sprite.
The sprite keeps its explicit
sizeWorld/saved size. Pixel frame dimensions do not imply a world-space resize; callupdateBillboardSpriteIndexwith bothframeandsizeWorldwhen that is desired. Existing flip state is preserved for non-degenerate UV ranges.