Printing to Xerox Workcentre 7855 fails when printing objects with less than 100% opacity

Printing to Xerox Workcentre 7855 fails when printing objects with less than 100% opacity.

EDIT: Reportedly the problem also affects some Canon printers using Canon Generic PCL6 Driver and some Kyocera printers.

Reasons to suspect a bug in Qt:

• reportedly non-Qt applications don't suffer from this issue.
   

Steps to reproduce:

1. Download the Windows drivers for a Xerox Workcentre 7855 from here: http://www.support.xerox.com/support/workcentre-7800-series/downloads/enca.html

• Choose 'built-in controller' from the 'Associated product' menu, and 'Windows 10 x64' from the 'Operating system' menu
• From the list of drivers choose 'Xerox Global Print Driver' -> Another page opens -> install PCL drivers using 'Install from Web' or 'PCL6 64-bit Driver - Download and Install'

2. Download and install 'GhostPCL' for viewing the printer driver raw output https://www.ghostscript.com/download/

3. Run the attached project, click the "Print" button, and choose "Print to File" after selecting the Xerox Workstation 7855 driver. Save as a .prn file. (See image attached)

4. Open the PCL file in GhostPCL:

• On command line: 'gpcl6win64.exe -dSAFER -dBATCH C:\PATH_TO_THE_PRINT\qtbugprint.prn'

5. Observe the results:
EXPECTED: A page full of randomly layered colorful rectangles
ACTUAL: Some pieces of colorful rectangles and white areas. The white areas are corrupt.

The corruption vanishes if you set the opacity to 255. (Line 44 in 'main.cpp', last argument of QColor)