3D-Viewer: Soldermask-free regions should not be rendered as bare copper

Description

#3862 (closed) (probably) introduced unrealistic 3D preview as can be seen in the attachment. In reality, all soldermask-free copper regions comes plated, I haven't seen bare copper exposed on any simple pcbs.

Steps to reproduce

1. Create any PCB and observe bare copper.

KiCad Version

Application: Pcbnew

Version: (5.99.0-3256-gd52c475b2), release build

Libraries:
	wxWidgets 3.0.5
	libcurl/7.71.0 OpenSSL/1.1.1g (Schannel) zlib/1.2.11 brotli/1.0.7 libidn2/2.3.0 libpsl/0.21.0 (+libidn2/2.3.0) libssh2/1.9.0 nghttp2/1.41.0

Platform: Windows 8 (build 9200), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Sep  8 2020 21:05:27
	wxWidgets: 3.0.5 (wchar_t,wx containers,compatible with 2.8)
	Boost: 1.73.0
	OCE: 6.9.1
	Curl: 7.71.0
	ngspice: 32
	Compiler: GCC 10.2.0 with C++ ABI 1014

Build settings:
	KICAD_SCRIPTING=ON
	KICAD_SCRIPTING_MODULES=ON
	KICAD_SCRIPTING_PYTHON3=OFF
	KICAD_SCRIPTING_WXPYTHON=ON
	KICAD_SCRIPTING_WXPYTHON_PHOENIX=OFF
	KICAD_SCRIPTING_ACTION_MENU=ON
	BUILD_GITHUB_PLUGIN=ON
	KICAD_USE_OCE=ON
	KICAD_SPICE=ON