CLIP_TYPE_NONE is valid if the clip is implemented by the scissor rect.
We always have a scissor rect and there's no way to draw outside of it.
In theory that means we can reset the clip to NONE at any point we
wish if we know nodes are contained inside a certain pixel-aligned
rectangle we can clip.
In practice that's probably quite hard...
This clip is different from "none" in that the bounds rect cannot be
ignored and that potential drawing outside the clip must be avoided.
In particular it means that clip nodes cannot be discarded if they
encompass the full clip region.
Fixes#6322
This heaves over an inital chunk of code from the Vulkan renderer to
execute shaders.
The only shader that exists for now is a shader that draws a single
texture.
We use that to replace the blit op we were doing before.
