eeschema: Plot to PDF - handling PNGs with transparency

Description

Eeschema perfectly renders transparent PNG images, but incorrectly plots them to pdf. Example files are tmp.zip.

Steps to reproduce

  1. Use some png image with transparency inside your schematics. You will get perfect results on screen;
  2. Plot your schematics to pdf;
  3. You will get distorted color png, with white background instead of transparency.

Input:
kicad_iqO8ExkNIL

Output:
FoxitReader_rXctOC9zeQ

Settings:
file-png_tNDwLjEtE9kicad_L7p6mwgIsl

KiCad Version

Application: Eeschema

Version: (5.99.0-3966-gf6ab7f4e78), 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 10 (build 18363), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Oct 11 2020 21:08:27
	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