Grid bitmap button: dropdown caret still visibile in background on Windows

Description

Steps to reproduce

KiCad Version

Application: Eeschema

Version: (5.99.0-8533-g07d4a28024), release build

Libraries:
	wxWidgets 3.1.4
	libcurl/7.74.0-DEV Schannel zlib/1.2.11

Platform: Windows 10 (build 19042), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Jan 20 2021 18:01:48
	wxWidgets: 3.1.4 (wchar_t,STL containers)
	Boost: 1.74.0
	OCC: 7.4.0
	Curl: 7.74.0-DEV
	ngspice: 32
	Compiler: Visual C++ 1928 without C++ ABI

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