Bug 894957 - Don't attempt to draw transparent text. r=roc

gfx/thebes/gfxFont.cpp

@@ -5748,10 +5748,18 @@ gfxTextRun::Draw(gfxContext *aContext, gfxPoint aPt, gfxFont::DrawMode aDrawMode
                  "GLYPH_PATH cannot be used with GLYPH_FILL, GLYPH_STROKE or GLYPH_STROKE_UNDERNEATH");
     NS_ASSERTION(aDrawMode == gfxFont::GLYPH_PATH || !aCallbacks, "callback must not be specified unless using GLYPH_PATH");
 
+    bool skipDrawing = mSkipDrawing;
+    if (aDrawMode == gfxFont::GLYPH_FILL) {
+        gfxRGBA currentColor;
+        if (aContext->GetDeviceColor(currentColor) && currentColor.a == 0) {
+            skipDrawing = true;
+        }
+    }
+
     gfxFloat direction = GetDirection();
 
-    if (mSkipDrawing) {
-        // We're waiting for a user font to finish downloading;
+    if (skipDrawing) {
+        // We don't need to draw anything;
         // but if the caller wants advance width, we need to compute it here
         if (aAdvanceWidth) {
             gfxTextRun::Metrics metrics = MeasureText(aStart, aLength,