5.99 Tuning diff pair doesn't work well with filleted tracks

Description

There had been reports about differential pair skew tuning destroying arcs in #6464 (closed) , and that issue was partially fixed. However when I was casually testing differential pairs with arcs I found that filleted tracks can't actually be tuned correctly.

(fillet radius: 1mm)

Also I can still destroy parts of the arc traces in the provided test case PCB (even when the actual trace is longer than your desired value). Not only that, it often leaves extra straight lines over the filleted tracks.

test_case.kicad_pcb

Steps to reproduce

fillet_diff_pair_skew_tuning

KiCad Version

Application: KiCad PCB Editor

Version: (5.99.0-11372-g1c28ea0577), release build

Libraries:
	wxWidgets 3.0.5
	libcurl/7.77.0 OpenSSL/1.1.1k zlib/1.2.11 zstd/1.5.0 libidn2/2.3.1 libpsl/0.21.1 (+libidn2/2.3.0) libssh2/1.9.0 nghttp2/1.43.0

Platform: Linux 5.10.42-1-MANJARO x86_64, 64 bit, Little endian, wxGTK, KDE, x11

Build Info:
	Date: Jul  8 2021 04:22:25
	wxWidgets: 3.0.5 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.76.0
	OCC: 7.5.0
	Curl: 7.77.0
	ngspice: 34
	Compiler: GCC 11.1.0 with C++ ABI 1016

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON