Allow for defining some render requests as unskipable

[Winter-Guerra]

FlightGoggles currently drops render requests if the render request queue is backed up. This is to ensure that a high rate render request feed from the client is not buffered in the incoming render queue. However, this can sometimes happen due to intermittent network latency. Sometimes, the user might want to render out some frames without allowing for packet drops. This requires adding a forceRender field to the render request message.

[Winter-Guerra]

Implemented in v1.7.0 release.