Add runtime detection for missing subpixel support in FreeType.

r291 added support for building on platforms where subpixel support
has been compiled out of FreeType. However, it turns out that there is
a common situation in which we build on platforms with subpixel
support, but have to run without it: people downloading Chromium from
the buildbots.

Because we this, we need a runtime solution. Modifying the conversion
routines for this seems suboptimal because the extra branches will
slow down everyone. So we probe subpixel support at runtime by trying
to install a low-pass filter. If we fail at this, then we map all the
SkScalerContext::Rec structures to remove subpixel mode.

http://codereview.appspot.com/98057


git-svn-id: http://skia.googlecode.com/svn/trunk@293 2bbb7eff-a529-9590-31e7-b0007b416f81

src/ports/SkFontHost_FreeType.cpp

@@ -73,6 +73,8 @@ static SkMutex      gFTMutex;
 static int          gFTCount;
 static FT_Library   gFTLibrary;
 static SkFaceRec*   gFaceRecHead;
+static bool         gLCDSupportValid;  // true iff |gLCDSupport| has been set.
+static bool         gLCDSupport;  // true iff LCD is supported by the runtime.
 
 /////////////////////////////////////////////////////////////////////////
 
@@ -86,6 +88,8 @@ InitFreetype() {
     // Setup LCD filtering. This reduces colour fringes for LCD rendered
     // glyphs.
     err = FT_Library_SetLcdFilter(gFTLibrary, FT_LCD_FILTER_DEFAULT);
+    gLCDSupport = err == 0;
+    gLCDSupportValid = true;
 #endif
 
     return true;
@@ -263,6 +267,17 @@ static void unref_ft_face(FT_Face face) {
 ///////////////////////////////////////////////////////////////////////////
 
 void SkFontHost::FilterRec(SkScalerContext::Rec* rec) {
+    if (!gLCDSupportValid) {
+      InitFreetype();
+      FT_Done_FreeType(gFTLibrary);
+    }
+
+    if (!gLCDSupport && rec->isLCD()) {
+      // If the runtime Freetype library doesn't support LCD mode, we disable
+      // it here.
+      rec->fMaskFormat = SkMask::kA8_Format;
+    }
+
     SkPaint::Hinting h = rec->getHinting();
     if (SkPaint::kFull_Hinting == h && !rec->isLCD()) {
         // collapse full->normaling hinting if we're not doing LCD