Invalidation panels can retain stale hittest data when caching is disabled/enabled

Changes to the hit test grid of an invalidation panel while caching is disabled won't be respected when caching is reenabled. This can cause interactive widgets such as buttons who were set to collapsed while caching was disabled to become clickable, even though they aren't being painted.

A potential fix here would be to clear the hit test grid in SInvalidationPanel::SetCanCache, assuming there aren't any knock-on effects.

See attached repro project

"Session Disconnected" UEFN

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