Add support for rounding bottom corners

This adds support for server-side rounded corners. The window decoration decides how much to round the corners and what corners should be rounded. Sub-surface corners are not rounded. In the future, we may add a wayland protocol to provide applications a way to ask the compositor to round the corners of their subsurfaces.

Corners are rounded and outlines are drawn using signed distance fields.

This requires some significant changes in the item renderer. I re-arranged a few things so the code is a bit nicer and it's actually something that can be pushed to production. However, in long term, more changes to the render node tree will be needed so more custom items can be rendered. That can wait until the Item tree is made persistent.

Depends on kdecoration!74 (merged)