vk_rasterizer: Implement constant attributes

Constant attributes (in OpenGL known disabled attributes) are not
supported on Vulkan, even with extensions. To emulate this behavior we
return zero on reads from disabled vertex attributes in shader code.
This has no caching cost because attribute formats are not dynamic state
on Vulkan and we have to store it in the pipeline cache anyway.

- Fixes Animal Crossing: New Horizons terrain borders