primitives: Snap icon vertices to device pixels after final transform

Volodymyr Zolotopupovrequested to merge
vvova/kirigami:icon-fractional-dpr-blur into master

Snap icon quad vertices in the shader after the final Qt Quick transform so they land on integer device pixels. Fixes subpixel blur and keeps icons sharp at fractional DPR.

Still testing, but looks good so far. It is same crispy result as when I switch Linear filtering to Nearest.

Before (scale 1.5)

Screenshot_20260419_170339.png

After (scale 1.5)

Screenshot_20260419_170400.png

Before (scale 1.5)

Screenshot_20260419_170416.png

After (scale 1.5)

Screenshot_20260419_170441.png

Before (scale 2.0)

Screenshot_20260419_170711.png

After (scale 2.0, has no regression)

Screenshot_20260419_170736.png

On 4k 2.5 scale also visible (top - after, bottom - before)

Screenshot_20260423_014738.png

Edited by Volodymyr Zolotopupov

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