This more or less rewrites this test so that it explicitly tests the
"interpolation" when a timeline loops, i.e. that if something occupies
the thread when a timeline was supposed to have looped, we end up in the
right place "in the middle" of the next timeline cycle.
The test more or less does this:
* Start a 3 second looping timeline
* Sleep so that we're in the middle of the first cycle
* Sleep again so that we end up in the middle of the next cycle
The semantics checked are that we see the following frames:
* The first frame with timestamp 0
* The second frame in the middle of the first cycle (timestamp ~= 1.5
sceonds)
* The third frame in the end of the first cycle (timestamp == 3.0
seconds)
* The fourth frame, first in the second cycle, with timestamp ~= 1.5
seconds)
This means we can increase the "grace period" to the double (from 0.5 s
to 1 s), while at the same time decrease the time spent running the test
(from 10 s to 4.5 s). This should hopefully make the test less flaky,
especially in slower runners, e.g. aarch64.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1751>
Without an associated actor, or explicit frame clock set, in the future
a timeline will not know how to progress, as there will be no singe
frame clock to assume is the main one. Thus, deprecate the construction
of timelines without either an actor or frame clock set.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1285
