3d viewer : The opacity of the solder mask color is not saved across sessions

Description

After setting an opacity different than 100% for the solder mask color, and reopening the PCB, the opacity is reset to 100%.

Steps to reproduce

• Open a PCB file and open the 3D viewer
• Open the color dialog of the solder mask : Preferences / Choose colors / Solder mask color...
• Adjust the opacity (right of this dialog) to a value different from 100%, and "OK"
• The 3D view is correctly displayed
• Close the 3D viewer and Pcbnew
• Re-open the pcb file and the 3D viewer
• The solder mask opacity has been reset to 100%

KiCad Version

Application: 3D Viewer

Version: (5.99.0-3346-g514da2f886), 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 7 (build 7601, Service Pack 1), 32 bit, Little endian, wxMSW

Build Info:
	Date: Sep 14 2020 22:30:15
	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