scene: Add pixel grid snapping debug pass

The debug shader is targeted to help with debugging blurriness when
using fractional scaling.

The shader works as follows:

- if the vertex coordinate has fractional part, the item will be
  highlighted with blue color
- if the texture coordinate (in device pixels) has fractional part,
  the item will be highlighted with red color

The shader can be toggled by setting the KWIN_SCENE_VISUALIZE=fractional
environment variable.