calcRenderVarFromDownloadedTimes

Recomputes meld/currentA/currentB/phaseAString/phaseBString for this layer from the global playhead (Timeline.currentPhaseA/Timeline.currentPhaseB/Timeline.dataMeld) mapped onto this layer's actually-downloaded reqLongs/reqStrings. Holds at lastMeld instead of advancing while paused or while viewportPlaybackReady is false, or if the global valid-time lists are empty.