OcdFileImport/Export: Handle v8 color overprint

Although the official docs do not mention the color overprint property,
the program dialog contains this setting. A quick experiment shows that
the checkbox controls a reserved struct member. This patch implements
color overprint import and export in OCD v8 format.

Author: lpechacek

src/fileformats/ocd_file_export.cpp

@@ -924,6 +924,7 @@ void OcdFileExport::exportSetup(OcdFile<Ocd::FormatV8>& file)
 
 			color_info->number = color->getId();
 			color_info->name = toOcdString(color->getName());
+			color_info->overprint = color->getKnockout() ? -1 : 0;
 			color_info->cmyk = ocd_cmyk;
 			++color_info;
 		}

src/fileformats/ocd_file_import.cpp

@@ -501,6 +501,7 @@ void OcdFileImport::importColors(const OcdFile<Ocd::FormatV8>& file)
 				// The color's CMYK was customized.
 				color->setCmyk(cmyk);
 		}
+		color->setKnockout(color_info.overprint != 0);		
 
 		if ((i == 0 && color->isBlack() && color->getName() == QLatin1String("Registration black"))
 		    || (!components.empty() && components.size() == num_separations

src/fileformats/ocd_types_v8.h

@@ -64,7 +64,7 @@ namespace Ocd
 	struct ColorInfoV8
 	{
 		quint16 number;
-		quint16 RESERVED_MEMBER;
+		qint16 overprint; // undocumented member, overprint: 0 = no, -1 = yes
 		CmykV8  cmyk;
 		PascalString<31> name;
 		quint8  separations[32];

test/file_format_t.cpp

@@ -1413,8 +1413,6 @@ void FileFormatTest::colorTest()
 		for (auto i = 0; i < original->getNumColorPrios(); ++i)
 		{
 			// >>>>>>>>> Temporary handling of OCD defects
-			if (*format_id == 'O' && original->getColorByPrio(i)->getKnockout() != copy->getColorByPrio(i)->getKnockout())
-				QEXPECT_FAIL("", "Overprint is not yet reliably maintained in OCD format", Continue);
 			if (*format_id == 'O' && i == 0 && original->getColorByPrio(0)->getName() != copy->getColorByPrio(0)->getName())
 				QEXPECT_FAIL("", "Registration black is not properly detected in absence of spot colors", Continue);
 			// <<<<<<<<< Temporary handling of OCD defects