Disable grims png compression

Grim by default used level 6 for compressing PDFs, which could take as
long as a second depending on the hardware. This caused a big delay when
taking a screenshot with grim, leading to a bad user experience.
This commit sets  the compression level to 0(uncompressed) radically
speeding up the process of taking a screenshot.

src/utils/screengrabber.cpp

@@ -34,6 +34,8 @@ void ScreenGrabber::generalGrimScreenshot(bool& ok, QPixmap& res)
     QProcess Process;
     QString program = "grim";
     QStringList arguments;
+    // Disable PNG compression
+    arguments << "-l" << "0";
     arguments << "-";
     Process.start(program, arguments);
     if (Process.waitForFinished()) {