Clean up Qt6 Windows main-window-flicker fix (#2484)

[dcwhite]

Context

PR #2463 includes a fix for #2484 (Qt6 Windows build: entire SCIRun app disappears momentarily when adding a saved snippet) — reportedly fixed by adding a dummy OpenGL window. Per review discussion, the fix landed but needs follow-up cleanup and cross-platform verification before it's considered done:

• src/Interface/Application/SCIRunMainWindow.cc:85 — "Need to test this on other platforms. Fix for Qt6 Windows build: entire SCIRun app disappears momentarily when you add a saved snippet #2484"
• src/Interface/Application/GuiApplication.cc:78 — "Should be a separate function explaining why this is here. Issue is Qt6 Windows build: entire SCIRun app disappears momentarily when you add a saved snippet #2484"
• Related comment: "I just love these funky Qt hacky fixes. SCIRun is littered with them."

Task

• Extract the dummy-OpenGL-window workaround into its own clearly named function with a comment explaining why it's needed (the Qt6/Windows flicker issue), instead of leaving it inline where the reason isn't obvious.
• Test the fix on macOS and Linux (only verified on Windows so far) to confirm it doesn't regress or misbehave on other platforms.
• Confirm the workaround is still needed / minimal — i.e. it's not silently masking a different Qt6 initialization ordering issue.

Related

• Qt6 Windows build: entire SCIRun app disappears momentarily when you add a saved snippet #2484 (original bug)
• PR Refactor: modernize cmake, boost, qt, and others to the latest or near latest #2463 (where the fix was introduced)