inactive-dim not respecting rounded corners

[quantenzitrone]

Platform

NixOS 22.05, but the issue should be platform independent

GPU, drivers, and screen setup

NVIDIA GeForce GTX 1050 Mobile && Intel HD Graphics 630

Environment

i3-gaps, but I think it should be environment independent

picom version

v9.1

Configuration:

exaggerated and simplified for demonstration

corner-radius = 100;
inactive-dim = 0.5;

Steps of reproduction

1. use this confguration
2. open 2 windows
3. the one currently not in focus should have this issue

Expected behavior

dim respects rounded corners and draws a rounded rectangle to dim the inactive windows

Current Behavior

dim draws a rectangle without rounded corners, like in this screenshot:

Other details

the relevant code might be located in this file: https://github.com/yshui/picom/blob/057a939431a5e856d001a5b2be9b9ba86738e829/src/render.c#L546-L583

so to fix this xcb_rectangle_t would have to be replaced by a shape that supports rounded corners. And at this point my knowledge of C and programming experience reaches its limits...