fix: prevent unnecessary rerendering of clusters when nothing has changed

[ixam1]

This PR fixes the unnecessary rerendering of clusters when nothing has changed.

Fixes #801 🦕

The fix is pretty simple, just compare the amount of clusters from before and after and if its the same -> dont render.

Correct me if I am wrong I am no expert in the algorithms used here, but this should cover all cases right? Since whenever clusters are changing they will either combine or decluster and in either way the amount of clusters will decrease or increase, so this simple check should work.

	without fix	with fix
fractional zoom with animation
normal zoom with animation
fractional zoom without animation

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[wangela]

Thanks @ixam1 for offering this fix; please make sure to sign the CLA so that doesn't block merging of your pull request.

[usefulthink]

This doesn't quite sit right with me. It seems to work, but I feel like checking the number of clusters as a proxy for "has anything changed" isn't accurate enough.

Why don't we fix the "changed" flag being returned instead, since obviously the algorithms don't properly signal changes to the markerclusterer?

[gopinath-sixt]

Thank you for these fixes, @ixam1 . Could you please let me know when we can expect to release them?