simplify temp file generation (port 6.0)

[weltkante]

Fixes #5717

Port from #5744

Proposed changes

• remove custom temp file name generation. It was using a newly allocated image handle to seed a random number generator, which is both unnecessary overhead and was leaking the image handle.

Customer Impact

• no more image handle leaks when assigning background images to list view

Regression?

• No

Risk

• probably none, unless someone relied on the temp file name patterns

Before

• leaking ~3 GDI handles every time the background images was assigned

After

• no image handle leak

Test methodology

• manual testing and observing GDI object count in task manager

Microsoft Reviewers: Open in CodeFlow

Microsoft Reviewers: Open in CodeFlow