Unbinding map does not adjust map_cache index

[techfg]

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Describe the bug
When unbinding a, imagemapster clears the map information. During this process, it adjusts its internal map_cache, however there is a bug when manipulating the indexes of the remaining maps in the cache.

This issue only manifests when there are multiple maps under imagemapster control. After the bug is triggered, imagemapster will not correctly manage its internal cache and behavior will be unreliable.

To Reproduce
Steps to reproduce the behavior:

1. Apply imagemapster to multiple maps (e.g. 0,1,2)
2. Check the internal data structure map_cache[i].index property and it will correctly indicate 0,1,2
3. Unbind a map "in the middle of the array of maps" (e.g. 1)
4. Check the internal data structure map_cache[i].index property and it will incorrectly indicate [0]=0,[1]=2 when it should be [0]=0,[1]=1

Expected behavior
The map_cache[i] index should correspond to the index of the map in the cache

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Additional context
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