This is a tricky topic, because it can make the clip bounds grow, so
previously we were trying to be careful.
However, this can cause perfectly trivial intersections to fail that are
caused by redraw diff regions.
And in the worst case, that means we offscreen in places where we
absolutely do not want to offscreen - in subtrees with subsurface nodes.
Fixes#6499
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.
