Live competition: Option not to display flight track before task start

[ArnulfKoch]

Feature request: There should be an option not to display the flight track before task start.

Reason: More clarity, e.g. if the flight track is already flown once at a ridge before take-off or if multiple take-offs are made in classic competitions

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[samuel-git]

I like the idea. I would opt for a sensible default like:
Show the whole track until the task starts, then show from the task start until the current glider position and hide the beginning.

@moldhouse what do you think?