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.
