pcbnew: intermeshing zones violate min clearance

Description

When creating intermeshing zones with clearance less than or equal to the minimum clearance of the board, the mesh that are drawn to fill the zones can violate the minimum clearance at zone corners.

Attached is a file showing the issue: intermeshing_zones.kicad_pcb.

General view, righ zone has priority set to 1, left zone is set to 0:

Measurement of gap between zones. Min clearance is set to 0.1mm on this design:

Zoom in that shows that the measurement points are inside the zones, therefore the gap is less than the displayed distance:

Admitedly, this is a small error, but it is still flagged by my board manufacturer as a violation of their rules for manufacturability.

Steps to reproduce

Create board with intermeshing zones, set their clearance to less than or equal to the minimum board clearance.

Workaround

Set zones clearance slighly above the minimum board clearance.

KiCad Version

Application: KiCad PCB Editor

Version: 6.0.2-378541a8eb~116~ubuntu20.04.1, release build

Libraries:
	wxWidgets 3.0.4
	libcurl/7.68.0 OpenSSL/1.1.1f zlib/1.2.11 brotli/1.0.7 libidn2/2.2.0 libpsl/0.21.0 (+libidn2/2.2.0) libssh/0.9.3/openssl/zlib nghttp2/1.40.0 librtmp/2.3

Platform: Linux 5.13.0-28-generic x86_64, 64 bit, Little endian, wxGTK, ubuntu, x11

Build Info:
	Date: Feb 11 2022 08:14:15
	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.71.0
	OCC: 7.5.2
	Curl: 7.81.0
	ngspice: 31
	Compiler: GCC 9.3.0 with C++ ABI 1013

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON