3d viewer: no copper layer unless "show copper thickness" is enabled

Description

Usually I have the "show copper thickness" option disabled (because it's slow to update the board). It seems now this option is required to see any copper layer at all (in non-raytracing mode).

"show copper thickness" disabled:

KiCad Version

Application: KiCad

Version: 5.99.0-unknown-r23674-43a4cf79, debug build

Libraries:
	wxWidgets 3.0.4
	libcurl/7.69.1 OpenSSL/1.1.1g-fips zlib/1.2.11 brotli/1.0.9 libidn2/2.3.0 libpsl/0.21.0 (+libidn2/2.3.0) libssh/0.9.5/openssl/zlib nghttp2/1.41.0

Platform: Linux 5.8.18-200.fc32.x86_64 x86_64, 64 bit, Little endian, wxGTK, gnome, wayland

Build Info:
	Date: Nov 12 2020 18:41:04
	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.69.0
	OCC: 7.4.0
	Curl: 7.69.1
	ngspice: 33
	Compiler: GCC 10.2.1 with C++ ABI 1014

Build settings:
	KICAD_SCRIPTING=ON
	KICAD_SCRIPTING_MODULES=ON
	KICAD_SCRIPTING_PYTHON3=ON
	KICAD_SCRIPTING_WXPYTHON=ON
	KICAD_SCRIPTING_WXPYTHON_PHOENIX=ON
	KICAD_SCRIPTING_ACTION_MENU=ON
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON
	KICAD_STDLIB_DEBUG=OFF
	KICAD_STDLIB_LIGHT_DEBUG=OFF
	KICAD_SANITIZE=OFF