Indicate whether a picture is coded as a frame, top field or bottom field.
For example, H.264 field_pic_flag equal to 0 corresponds to
AV_PICTURE_STRUCTURE_FRAME. An H.264 picture with field_pic_flag
equal to 1 and bottom_field_flag equal to 0 corresponds to
AV_PICTURE_STRUCTURE_TOP_FIELD.
Indicate whether a picture is coded as a frame, top field or bottom field.
For example, H.264 field_pic_flag equal to 0 corresponds to AV_PICTURE_STRUCTURE_FRAME. An H.264 picture with field_pic_flag equal to 1 and bottom_field_flag equal to 0 corresponds to AV_PICTURE_STRUCTURE_TOP_FIELD.