Screen Probes and Specular Firefly Flickering near screen edges

Lumen's rough specular indirect lighting buffer can have fireflies along the bottom and right edges of the screen.

Also tested in //UE5/Main, CL: 35177423

1. Download the Licensee's test project and open
https://udn.unrealengine.com/s/contentdocument/069QP00000FQFHJYA5
2. Don't move the camera at all during the next steps
3. Press play to enter PIE.
4. Press "3"
5. Observe that the console command "r.Lumen.ScreenProbeGather.DownsampleFactor 64" was run automatically
6. Observe the fireflies on the right-hand side of the screen
7. Press "Esc" to exit PIE.
8. Observe the fireflies on the bottom of the screen.
9. Run the console command "vis Lumen.ScreenProbeGather.RoughSpecularIndirect"
10. Observe the same fireflies in this internal texture.