Improvements to experimental image viewer

[hjiangsu]

Pull Request Description

This PR fixes a few issues with the experimental image viewer:

• Added ability to pan image when zoomed
• Fixed shadow gradient overlapping with image when full-screen
• Fixed an issue where double-tapping to zoom would not reset the zoom

Issue Being Fixed

Issue Number: N/A

Screenshots / Recordings

Checklist

• If a new package was added, did you ensure it uses an appropriate license and is actively maintained?
• Did you use localized strings (and added appropriate descriptions) where applicable?
• Did you add semanticLabels where applicable for accessibility?