vulkan: Try really hard to use clear

vulkan: Try really hard to use clear

Using clear avoids the shader engine (see last commit), so if we can get
pixels out of it, we should.

So we detect the overlap with the rounded corners of the clip region and
emit shaders for those, but then use Clear() for the rest.

With this in place, widget-factory on my integrated Intel TigerLake gets
a 60% performance boost.