fix: importing via magick++ for version >= 7.0.1

There was an undefined behavior due to multiple increment
of same variable in multiple function arguments.

fix #3331

synfig-core/src/modules/mod_magickpp/mptr_magickpp.cpp

@@ -128,33 +128,43 @@ magickpp_mptr::get_frame(synfig::Surface& surface, const synfig::RendDesc& /*ren
 		const auto height = image.size().height();
 		surface.set_wh(width, height);
 
-		image.type(Magick::TrueColorType);
-// Sadly Magick++ API is a mess with namespaces...
-{
-	using namespace Magick;
-	using namespace MagickCore;
+#if MagickLibVersion >= 0x701 // MAGICKCORE_CHECK_VERSION(7,0,0) does not work!
+		const bool has_alpha = image.alpha();
+#else
+		const bool has_alpha = image.matte();
+#endif
 
+		if (has_alpha)
 #if MagickLibVersion >= 0x701 // MAGICKCORE_CHECK_VERSION(7,0,0) does not work!
-		bool has_alpha = image.alpha();
+			image.type(Magick::TrueColorAlphaType);
 #else
-		bool has_alpha = image.matte();
+			image.type(Magick::TrueColorMatteType);
 #endif
+		else
+			image.type(Magick::TrueColorType);
+
 		const auto packet = image.getConstPixels(0, 0, width, height);
 
 		if (!packet) {
 			synfig::error(_("Magick++ importer: couldn't get pixel packet"));
 			return false;
 		}
+// Sadly Magick++ API is a mess with namespaces...
+{
+	using namespace Magick;
+	using namespace MagickCore;
+
 
 		constexpr synfig::Color::value_type factor = QuantumRange;
 
 		for (size_t y = 0, i = 0; y < height; ++y) {
 			for (size_t x = 0; x < width; ++x, ++i) {
 #if MagickLibVersion >= 0x701 // MAGICKCORE_CHECK_VERSION(7,0,0) does not work!
 				if (has_alpha)
-					surface[y][x] = synfig::Color(packet[i++] / factor, packet[i++] / factor, packet[i++] / factor, packet[i] / factor);
+					surface[y][x] = synfig::Color(packet[i] / factor, packet[i+1] / factor, packet[i+2] / factor, packet[i+3] / factor);
 				else
-					surface[y][x] = synfig::Color(packet[i++] / factor, packet[i++] / factor, packet[i] / factor, 1.);
+					surface[y][x] = synfig::Color(packet[i] / factor, packet[i+1] / factor, packet[i+2] / factor, 1.);
+				i += has_alpha ? 3 : 2;
 #else
 				const auto& color = packet[i];
 				if (has_alpha)