Micro-optimisation: Avoid Activator.CreateInstance when creating MediaWithCrops

[patrickdemooij9]

Prerequisites

• I have added steps to test this contribution in the description below (I think the current tests are coveraging this case already)

Description

As the old comment noted, the creation of the MediaWithCrops is not cached and therefore slow and expensive. Other places in the codebase are already caching this type of creation. My code now also adds that system here. I've also added a benchmark to showcase the change:

Method	Mean	Error	StdDev	Ratio	RatioSD	Gen0	Allocated	Alloc Ratio
'After: single item (compiled delegate, warm)'	28.15 ns	15.72 ns	0.862 ns	1.00	0.04	0.0037	48 B	1.00
'After: ten items (compiled delegate, warm)'	266.26 ns	80.38 ns	4.406 ns	9.47	0.28	0.0363	480 B	10.00
'Before: single item (Activator.CreateInstance)'	333.43 ns	210.53 ns	11.540 ns	11.85	0.47	0.0325	424 B	8.83
'Before: ten items (Activator.CreateInstance)'	2,978.63 ns	898.57 ns	49.254 ns	105.89	3.16	0.3261	4240 B	88.33

[github-actions]

Hi there @patrickdemooij9, thank you for this contribution! 👍

While we wait for one of the Core Collaborators team to have a look at your work, we wanted to let you know about that we have a checklist for some of the things we will consider during review:

• It's clear what problem this is solving, there's a connected issue or a description of what the changes do and how to test them
• The automated tests all pass (see "Checks" tab on this PR)
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• Avoids creating breaking changes; note that behavioral changes might also be perceived as breaking
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[AndyButland]

Thanks again @patrickdemooij9 - all looks good. I just pushed a little tidy-up and again some further unit tests, to ensure we have good coverage over this optimisation.

[AndyButland]

Cherry-picked to v17/dev.