Add "low power" shaders for shadowedrectangle that can be used on very low power hardware

As it turns out, the PinePhone has a GPU that can process only about 14 instructions per pixel when we want to run things at 60 fps. Since it also lacks hardware fallback of some key operations that are used a lot in the distance field calculations, the current shaders do not perform well on the PinePhone. This adds a set of greatly simplified shaders that remove some of the rendering details but do perform reasonably on the PinePhone.

These shaders are used when the KIRIGAMI_LOWPOWER_HARDWARE environment variable is set.