We added a source that allows mixed placeholder and non-placeholder
periods, but have no way to denote that in the Timeline because the
placeholder flag only exists on Window level. This causes a bug if
the first item in a concatenation has a window-period offset and the
player can't detect whether it's still a placeholder or not.
Adding this flag to Period allows the player to detect this reliably.
In addition we need to make sure that re-resolving pending positions
only happens for the first placeholder period where the window-offset
can actually change. As all subsequent periods have to start at position
0, so they don't need to be re-resolved (and shouldn't).
PiperOrigin-RevId: 367171518
