Fix bug with stacking sensor(#5790)

[x-mug]

grid sensor and all PNG compression used will face this bug when channel number is larger than 3 && used stacking observation (stack > 1)

Proposed change(s)

Describe the changes made in this PR.

The prepared empty PNG will be multiplied based on the channel number used

Useful links (Github issues, JIRA tickets, ML-Agents forum threads etc.)

https://github.com/Unity-Technologies/ml-agents/issues/5790

Types of change(s)

• [x] Bug fix
• [ ] New feature
• [ ] Code refactor
• [ ] Breaking change
• [ ] Documentation update
• [ ] Other (please describe)

Checklist

• [x] Added tests that prove my fix is effective or that my feature works
• [ ] Updated the changelog (if applicable)
• [ ] Updated the documentation (if applicable)
• [ ] Updated the migration guide (if applicable)

Other comments

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