[plugins/qpa] Add support for QT_WAYLAND_FORCE_DPI env variable

Summary:
If a user specifies the QT_WAYLAND_FORCE_DPI env variable, KWin uses
it to force a logicalDPI, just like QtWayland.

Test Plan: Normally sized window decorations

Reviewers: #plasma

Subscribers: plasma-devel

Projects: #plasma

Differential Revision: https://phabricator.kde.org/D1605

CMakeLists.txt

@@ -3,7 +3,7 @@ set(PROJECT_VERSION "5.6.90")
 set(PROJECT_VERSION_MAJOR 5)
 
 cmake_minimum_required(VERSION 2.8.12 FATAL_ERROR)
-set(QT_MIN_VERSION "5.4.0")
+set(QT_MIN_VERSION "5.5.0")
 set(KF5_MIN_VERSION "5.12.0")
 
 set(CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules ${CMAKE_MODULE_PATH} )

plugins/qpa/screen.cpp

@@ -62,5 +62,15 @@ QPlatformCursor *Screen::cursor() const
     return m_cursor.data();
 }
 
+QDpi Screen::logicalDpi() const
+{
+    static int force_dpi = qEnvironmentVariableIsSet("QT_WAYLAND_FORCE_DPI") ? qEnvironmentVariableIntValue("QT_WAYLAND_FORCE_DPI") : -1;
+    if (force_dpi > 0) {
+        return QDpi(force_dpi, force_dpi);
+    }
+
+    return QPlatformScreen::logicalDpi();
+}
+
 }
 }

plugins/qpa/screen.h

@@ -47,6 +47,7 @@ public:
     QImage::Format format() const override;
     QSizeF physicalSize() const override;
     QPlatformCursor *cursor() const override;
+    QDpi logicalDpi() const override;
 
 private:
     KWayland::Client::Output *m_output;