Turn on glyph caching by default when using OpenGL

Can now be turned off with a SAL_DISABLE_GLYPH_CACHING environment variable.

Change-Id: I7ac14d72dc0f85c0682d92492eb96bec1d207609
(cherry picked from commit 9a68eb9c)
Reviewed-on: https://gerrit.libreoffice.org/18230Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>

vcl/README.vars

@@ -20,4 +20,4 @@ OpenGL
 ------
 SAL_FORCEGL - force enable OpenGL
 SAL_WITHOUT_WIDGET_CACHE - disable LRU caching of native widget texutres
-SAL_ENABLE_GLYPH_CACHING - render glyphs to textures and use those, Windows only, WIP, broken
+SAL_DISABLE_GLYPH_CACHING - don't render glyphs through OpenGL textures

vcl/win/source/gdi/winlayout.cxx

@@ -1667,7 +1667,7 @@ void UniscribeLayout::DrawTextImpl(HDC hDC) const
 
 bool UniscribeLayout::CacheGlyphs(SalGraphics& rGraphics) const
 {
-    static bool bDoGlyphCaching = (std::getenv("SAL_ENABLE_GLYPH_CACHING") != NULL);
+    static bool bDoGlyphCaching = (std::getenv("SAL_DISABLE_GLYPH_CACHING") == NULL);
 
     if (!bDoGlyphCaching)
         return false;