TemporalBlur: allow 0 jitter and fix small bug

[Yvee1]

Setting jitter to 0 didn't work before because Vector2.uniformRing then tries to generate a number with squared norm strictly less than 0, and gets struck in an infinite loop.

Another small issue I found was that samples - 1 different timestamps were used and one was rendered twice. For illustration:

Before:

After:

[edwinRNDR]

Much appreciated!