Add other font size scaling

Minor fixes:
- Fixes some incorrect cast
- fixes some incorrect comments

rtgui/lockablecolorpicker.cc

@@ -48,7 +48,20 @@ void LockableColorPicker::updateBackBuffer ()
         Pango::FontDescription fontd = pangoContext->get_font_description();
         // set font family and size
         fontd.set_family(options.CPFontFamily == "default" ? "sans" : options.CPFontFamily);
-        fontd.set_size((options.CPFontFamily == "default" ? 8 : options.CPFontSize) * Pango::SCALE);
+        const int fontSize = options.CPFontFamily == "default" ? 8 : options.CPFontSize; // pt
+        // Converting font size to "px" based on DPI and scale
+#ifndef __APPLE__
+        const double fontScale = RTScalable::getDPI() / RTScalable::pangoDPI
+            * static_cast<double>(RTScalable::getScale()); // Refer to notes in rtscalable.h
+#else
+        // On MacOS, font is already scaled by the System library
+        // Refer to https://gitlab.gnome.org/GNOME/gtk/-/blob/gtk-3-24/gdk/quartz/gdkscreen-quartz.c
+        const double fontScale = 1.;
+#endif
+        const double absoluteFontSize = static_cast<double>(fontSize) * fontScale; // px
+        // Absolute size is defined in "Pango units" and shall be multiplied by
+        // Pango::SCALE from "px":
+        fontd.set_absolute_size (absoluteFontSize * static_cast<double>(Pango::SCALE));
         fontd.set_weight(Pango::WEIGHT_NORMAL);
         pangoContext->set_font_description (fontd);