rsx/overlays: Misc Improvements

[kd-11]

• Unify overlay rendering shaders to reduce the headache of maintaining 2 separate paths
• Add support for disabling vertex snap on the overlays. This fixes rendering the round parts of rounded rectangles.
• Fix grid snap code which was broken for years due to a misplaced bracket 🙈
• Force disable the rounded rectangles feature on mac OS which doesn't support triangle fans. A workaround will be provided at a later time.

[Megamouse]

hmm maybe also add a comment. otherwise noone knows why it's ifdefed

[kd-11]

hmm maybe also add a comment. otherwise noone knows why it's ifdefed

I think the __APPLE__ part says it all 😆
It's temporary anyway, I'll get to it when I can build macos without tearing my hair out.

[Megamouse]

maybe show comparisons. too lazy to take a look lmao

[kd-11]

After: