mov: Support default-base-is-moof.

default-base-is-moof shall be set to track fragments compatible with DASH
Media Segments. So, this is a fundamental support for ISOBMFF ver. DASH.
This is meaningful only when base-data-offset-present is absent and two or
more track fragments are present in a movie fragment.

Signed-off-by: Martin Storsjö <martin@martin.st>