Tatsuyuki Ishi authored Oct 01, 2021 and Noah Davis committed Oct 11, 2021

QPainter's auto-scaling is prone to off-by-one rounding errors and draws on
fractional coordinates. With this change, we paint on a 1x DPR QPainter and
scale the shadow offset and strength manually based on DPR.

This resolves an issue with resulting in seams on the right and bottom
edges of a menu due to shadow boundaries being off-by-one.

BUG: 418166

v2: remove unrelated formatting changes
v3:
- move the DPR helper to ShadowHelper
- retrieve the DPR from the widget instead of the global QGuiApplication
- added BUG reference