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](/uploads/6d17d6f2d5952690fb445ab06c365306/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:
Output:
Settings:
# KiCad Version
<!-- Copy version information (from main menu Help->About KiCad ->Copy Version Info) and paste it between the triple backticks below to preserve the formatting. -->
```
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
```
issue