Send the work we have so far to the GPU, but there may be more under way.
A fence won't be generated automatically to protect this work, but
there may be one if getCurrentFence or acquireCurrentFence
was called
NewFrameIdx
Same as FlushOnly + fences the data so that VaoManager::mDynamicBufferCurrentFrame can be incremented and waitForTailFrameToFinish works without stalling.
It's like forcing the end of a frame without swapping windows, e.g. when waiting for textures to finish streaming.