Fixed background transparency with Images

[yashlamba]

Co-authored-by: TwoTicks [email protected]

Resolves #5175
Continues #5225

@two-ticks suggested the globalAlpha implementation in #5225, this PR implements it.

Changes:

Example
Replicated the example from #5225, to demonstrate consistency with the other solution.

• Setting alpha in background now works with images.
• Alpha parameter range is consistent with the docs (0-255), wanted to point this out since globalAlpha uses 0-1 range.

Screenshots of the change:

PR Checklist

• npm run lint passes
• Inline documentation is included / updated
• Unit tests are included / updated

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[yashlamba]

@two-ticks, added you as a co-author, I hope you don't mind. In case you want me undo that, please let me know :)

[limzykenneth]

Looks good. Thanks!