So now we can have access to Skia SkPaint instance directly from
Aseprite UI code (and set the shader, and other properties that are
not yet well-wrapped in laf-os).
We've simplified ui::CheckeredDrawMode with
ui::set_checkered_paint_mode() function, as now the checkered
background shader is stored in ui::Paint (SkPaint) instead of
ui::Graphics state.
