Change Usd_Clip to hold a shared pointer to the array of time mappings

[marktucker]

Description of Change(s)

Change Usd_Clip to hold a shared pointer to the array of time mappings.
Set up this array in Usd_ClipSet where all the Usd_Clip objects are
allocated. Prevents N^2 memory usage on the number of time mappings in
a clip set.

Fixes Issue(s)

• Value Clips memory usage grows with the square of the number of "times" entries #1774

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[jilliene]

Filed as internal issue #USD-7226

[pberto]

@marktucker nice improvement here.

Since you brought this up, how are you dealing with the other issues with memory limitation and clips? I mean when you actually do have a lot of data for each frame, and not just a box, it is possible to incur in going out of memory when usdStitchClip will go through all the data. At the present state the only solution that I could find was to make a clip with few frames and then edit (by scripting or even by hand) the timecodes in the clip file and in the topology file.