Lazily reset `start_time` in VisualBell's `completed` method

Fixes #416. Here's what I think is happening: at short `duration`s, the
VisualBell's [`completed` check](https://github.com/jwilm/alacritty/blob/3ce6e1e4b2924b0d432cbb3e62b4bbef88dd38ff/src/term/mod.rs#L377)
is returning `true` before we've actually had a chance to draw the "normal"
background color. I thought about driving this condition off of whether or not
`intensity` returns 0.0, but we may still miss a draw, I think. Perhaps the best
way to tackle this is to let `completed` lazily reset the VisualBell's
`start_time` (something @jwilm asked about when this feature originally landed),
and to only return `true` when the VisualBell's `start_time` is `None`. This
should effectively delay the final draw until after the VisualBell completes.

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