Fix timing issues with accumulation motion blur

[pmavridis]

Purpose of this PR

As reported by the recorder team, there were some timing issues with the accumulation motion blur (the camera transitions in recorded timelines were not consistent with the non-mb case).

This problem was created from two different aspects of our first implementation:

1. The use of Time.timescale did not "play nice" with timelines. In the new implementation we scale manually the Time.captureDeltaTime and the Time.fixedDeltaTime parameters instead of using Time.timescale.

2. There was some complex logic that was manipulating the Time.timescale per frame. This was done to avoid sampling frames with zero contribution when the shutter interval is < 1 and for centering the MB. However, my tests has shown that this was also affecting the timing. The new implementation removed this complex logic (so we are a bit less efficient when shutter interval is < 1) and the mb is now forward. The code is now more simple/easy to understand and the timing is consistent with non-mb.

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Testing status

• @jeanblouin already had an early look at this branch and he reported that it fixes the issues they were having with the recorder.
• I have added some falling objects (with rigid body dynamics) in the new HDRP template scene and recorded with and without mb. The timings (the frame in which the objects hit the ground) were consistent.
• I have recorded the timelines that are available in the HDRP template and the timing were also consistent with the non-mb case.