SG-40373: Alt+Shift key up doesn't clear Alt modifier (#879)

[williamwira]

Linked issues

Summarize your change.

This update includes Alt in handling that already exists for clearing modifiers after a modifier mismatch occurs.

Describe the reason for the change.

Alt + Shift modifier key-down was incorrectly identifying as Meta + Shift on key-up. As a result, Alt state continued after key up, which caused operators to think RV was hung -- spacebar wouldn't play/pause, dragging cursor on screen wouldn't scrub timeline, etc.

Describe what you have tested and on which operating system.

Reproduced Alt+Shift issue on Linux RHEL 9.4 and RHEL 7.2, using:

• Latest OpenRV main branch
• RV 2023.0.3
• RV 7.2.6

Confirmed it did not happen on MacOS Sequoia.

Add a list of changes, and note any that might need special attention during the review.

If possible, provide screenshots.