In osu-stable, storyboard intros start from the first command, but in
the case of storyboard drawables which have an initial hidden state, all
commands before the time at which they become visible (ie. the first
command where `Alpha` increases to a non-zero value) are ignored.
This brings lazer in line with that behaviour. It also removes several
unnecessary LINQ calls.
Note that the alpha check being done in its own pass is important, as
it must be the "minimum present alpha across all command groups,
including loops". This is what makes the implementation slightly
complex.
Closes#11981.
