Add icon for the Decal node

[YeldhamDev]

Very simple design, but I think it gets the point across.

Depends on #37861 being merged first, of course.

[clayjohn]

Oooh, I really like it! Something like this would be really nice for the new *SkyMaterials. If you are interested in making icons, PanoramaSkyMaterial ProceduralSkyMaterial and PhysicalSkyMaterial all need some.

[reduz]

I have to admit its nice, but it took me a while to figure out what it is... I was thinking a stamp might be more obvious.

[akien-mga]

I like the "sticker" design.

The stamp is also nice, but my first thought was that it was a chess pawn, so I'm not sure it will be clear for everyone.

Before seeing any proposal, my idea was an icon that would be like paint splatter/splash, but it might not work well in our small icon size.

[YeldhamDev]

Just reminding that icon nodes are presented very small, so the less detail they have, the better.

[RibalGZ]

I think the stamp is not a good idea, it's commonly used for cloning tools.

[akien-mga]

Thanks!