blur: add clipping region

[the-eater]

Currently blur does not respect the clipping region

So when blur is enabled on an element, and a clipping region is applied, the clipping region is filled with blur.

My PR on swayfx is affected by this as shown by discord user ahf

Both the top and bottom parts shouldn't be covered by blur and simply show the contents of the main buffer, however the border is rendered after the content, and overlays it with the blur.

Adding this clipping code fixes it.

[WillPower3309]

Could I see a screenshot of what it looks like after the PR is applied? Logic looks good but want to see it as well

[the-eater]

Both of these are made with the current head in my branch in swayfx, no changes will be needed there

With unpatched scenefx:

With this patch

[ErikReider]

Both of these are made with the current head in my branch in swayfx, no changes will be needed there

Are you applying the blur to the border scene_buffer? I'm not really getting that's being shown here

[the-eater]

@ErikReider I'm applying blur to the scene rect's of the border's and titlebar, as seen here

the-eater/swayfx@7eed52a

The top and bottom border in screenshots are made taller to accommodate the border radius, and then a clipping region is used to still display the content behind it.

e.g. without the patch but also without blur it displays fine

[ErikReider]

Otherwise, LGTM

[WillPower3309]

Just one comment, thanks!

[the-eater]

clipping in favor of #149