Analyze cause of disposed image during DPI change event

[HeikoKlare]

I ran into a broken UI after processing a DPI change event because a disposed image was tried to be set, see: eclipse-platform/eclipse.platform.swt#2432

It shall be analyzed why a disposed image can occur there and where/how/if to fix this.

[HeikoKlare]

Cause was found and documented in:

• [Win32] Clean up disposed images when retrieving image list handle #2432 eclipse-platform/eclipse.platform.swt#2447