Fix RoundedCornersEffect

[lenemter]

Third time's a charm

Rename .vert shaders to .frag because well, these are fragment shaders, not vertex shaders.

Fixes the strange offsets in shader code by correctly calculating offsets using clutter_actor_box_enlarge_for_effects which is mutter private api. This works with and without fractional scaling which was the flaw of my previous attempts

[leolost2605]

The corners look a lot better already but I'm still noticing another thing. On main with scale of 1.5 it looks like this

which has a corner radius of 5 pixels (if you ignore the semi transparent additional pixel btw is this intended?) which make sense being 3 * 1.5 rounded up.

With this branch however it has a corner radius of 4? Is that intended? It looks a lot less round in practice

Btw I don't know much about corner radii and where you start counting them etc. so correct me if I'm wrong somewhere I just noticed the difference here :)

Also is there a specific issue this branch fixes so that I can check that?

[lenemter]

@leolost2605 Gtk4 tooltips seem to have the same issue, so I guess 4px is intended..?

[leolost2605]

@leolost2605 Gtk4 tooltips seem to have the same issue, so I guess 4px is intended..?

Ah all right that's good to know but IMO it looks better there because on both sides it is fully opaque whereas in the screenshot from this branch only one side is really opaque and the other quite transparent.

Also another thing I noticed is that there are different corners on the top and bottom (this is at 1.25x scaling):

I don't know, if you prefer we can merge this for now and then continue fine tuning later since it is very self contained.
But can we maybe split out the file renaming to a separate commit/PR?