A strip was wrongly deemed to be opaque (and thus strips below it
were skipped from rendering), if the strip content was opaque, but
it gained transparency via presence of a Mask modifier.
While at it, I also noticed that a strip could have been wrongly deemed
opaque when it had a Multiplier < 1.0 with "multiply alpha" option
checked under strip Color settings. So fixed that too.
The whole logic of that factored out into is_opaque_alpha_over function
to make the call site clearer.
