Disable and hide the pixmap cache feature

lib/RELEASE-NOTES

@@ -20,6 +20,10 @@
   See chapter 3.9.1.1 "Dashes and Line Breaks" of the User Guide and
   "Caveats when upgrading from earlier versions to 2.3.x" below.
 
+* The preference "Tools->Preferences->Look&Feel->Screen Fonts->Use
+  pixmap cache to speed up font rendering" is not available anymore.
+  It was of dubious value and led to rendering issues.
+
 * The following UI translations were dropped, because the lack of translation
   maintenance:  Russian, Danish, Greek, Serbian, Galician, Catalan, Romanian,
   Dutch.
@@ -87,6 +91,8 @@
 * \screen_zoom
   This is now independent of the value set via buffer-zoom[-in|-out].
 
+* \use_pixmap_cache
+  The value of this variable is now ignored.
 
 !!!The following new LyX functions have been introduced in 2.3:
 

lib/doc/UserGuide.lyx

@@ -46198,24 +46198,6 @@ reference "subsec:Document-Font"
 .
 \end_layout
 
-\begin_layout Standard
-With the option 
-\family sans
-Use pixmap cache to speed up font rendering
-\family default
- enabled, \SpecialChar LyX
- needs to redraw the screen less often.
- This results in better performance, especially on slow systems.
- On the other hand, the characters might look more fuzzy on screen.
- So whether you enable this or not depends on whether you prefer speed over
- aesthetics.
- Note that the Pixmap Cache is only available and useful under Mac
-\begin_inset space ~
-\end_inset
-
-OS and Windows.
-\end_layout
-
 \begin_layout Subsection
 Colors
 \begin_inset Index idx

src/frontends/qt4/GuiPainter.cpp

@@ -50,7 +50,7 @@ const int Painter::thin_line = 1;
 
 GuiPainter::GuiPainter(QPaintDevice * device, double pixel_ratio)
 	: QPainter(device), Painter(pixel_ratio),
-	  use_pixmap_cache_(lyxrc.use_pixmap_cache && USE_PIXMAP_CACHE)
+	  use_pixmap_cache_(false)
 {
 	// new QPainter has default QPen:
 	current_color_ = guiApp->colorCache().get(Color_black);

src/frontends/qt4/GuiPrefs.cpp

@@ -1137,7 +1137,7 @@ void PrefScreenFonts::updateRC(LyXRC const & rc)
 #if defined(Q_WS_X11) || defined(QPA_XCB)
 	pixmapCacheCB->setEnabled(false);
 #endif
-
+	pixmapCacheCB->hide();
 }