Improve HiDpi handling

Add code stolen from QBitTorrent and Qt Creator that properly enables
HighDpi handling.

The rules are:

1/ nothing needs to be done for macOS and Qt >= 6

2/ HighDpi support is set by default starting with Qt 5.6.

3/ Support for fractional scaling is added for Qt >= 5.14.

Hopefully fixes bug #12060.

(cherry picked from commit d7a4f47e7d)
(cherry picked from commit f92f2f95ff)

src/frontends/qt4/GuiApplication.cpp

@@ -63,6 +63,7 @@
 #include "support/convert.h"
 #include "support/debug.h"
 #include "support/ExceptionMessage.h"
+#include "support/environment.h"
 #include "support/FileName.h"
 #include "support/filetools.h"
 #include "support/ForkedCalls.h"
@@ -194,6 +195,24 @@ frontend::Application * createApplication(int & argc, char * argv[])
 	AllowSetForegroundWindow(ASFW_ANY);
 #endif
 
+
+// Setup high DPI handling. This is a bit complicated, but will be default in Qt6.
+// macOS does it by itself.
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0) && !defined(Q_OS_MAC)
+#if QT_VERSION >= QT_VERSION_CHECK(5, 6, 0)
+    // Attribute Qt::AA_EnableHighDpiScaling must be set before QCoreApplication is created
+    if (getEnv("QT_ENABLE_HIGHDPI_SCALING").empty()
+		&& getEnv("QT_AUTO_SCREEN_SCALE_FACTOR").empty())
+        QApplication::setAttribute(Qt::AA_EnableHighDpiScaling, true);
+#endif
+
+#if QT_VERSION >= QT_VERSION_CHECK(5, 14, 0)
+    // HighDPI scale factor policy must be set before QGuiApplication is created
+    if (getEnv("QT_SCALE_FACTOR_ROUNDING_POLICY").empty())
+        QApplication::setHighDpiScaleFactorRoundingPolicy(Qt::HighDpiScaleFactorRoundingPolicy::PassThrough);
+#endif
+#endif
+
 	frontend::GuiApplication * guiApp = new frontend::GuiApplication(argc, argv);
 	// I'd rather do that in the constructor, but I do not think that
 	// the palette is accessible there.

status.23x

@@ -34,8 +34,11 @@ What's new
 - Other than directly inserted, now references can also be pasted as insets
   into an equation, instead of simply as text (bug 11539). 
 
+- Add support for High DPI screens in Windows and Linux (bug 12060).
+
 - Add Qt platform name in About dialog.
 
+
 * DOCUMENTATION AND LOCALIZATION