* asyncResourceGatherer: start the asyncLoop at the same time as gather
This is a prerequesit for labels beeing drawn, while backgrounds are
note ready yet.
* core: allow immediate rendering even when backgrounds are not gathered yet
Note:
We don't really need to call `asyncResourceGatherer::apply` in the
`renderLock` function, since it will get called by a call to
`asyncResourceGatherer::getAssetById` anyways.
* background: render color rectangle when asset is not ready yet
* config: add general:immediate_render config option
* core: use the --immediate-render flag in attemptRestoreOnDeath
