Instead of manually triggering `ScriptThread::update_the_rendering`,
have animated images trigger rendering updates via the `ScriptThread`
event loop. This should result in fewer calls to
`ScriptThread::update_the_rendering`.
Testing: This should not change behavior and is thus covered by existing
tests.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Mukilan Thiyagarajan <mukilan@igalia.com>