b=782185 provide a gfxXlibSurface method to indicate when cairo's extend_pad will be slow r=joe

--HG--
extra : transplant_source : M%D2%05%3A%C1%18W%BD.%BA8%18%BD%F9%F9%AEu%EEX%CE

gfx/thebes/gfxDrawable.cpp

@@ -73,15 +73,7 @@ PreparePatternForUntiledDrawing(gfxPattern* aPattern,
             // Cairo, and hence Gecko, can use RepeatPad on Xorg 1.7. We
             // enable EXTEND_PAD provided that we're running on a recent
             // enough X server.
-
-            gfxXlibSurface *xlibSurface =
-                static_cast<gfxXlibSurface *>(currentTarget);
-            Display *dpy = xlibSurface->XDisplay();
-            // This is the exact condition for cairo to avoid XRender for
-            // EXTEND_PAD
-            if (VendorRelease(dpy) >= 60700000 ||
-                VendorRelease(dpy) < 10699000) {
-
+            if (static_cast<gfxXlibSurface*>(currentTarget)->IsPadSlow()) {
                 bool isDownscale =
                     aDeviceToImage.xx >= 1.0 && aDeviceToImage.yy >= 1.0 &&
                     aDeviceToImage.xy == 0.0 && aDeviceToImage.yx == 0.0;

gfx/thebes/gfxXlibSurface.h

@@ -79,6 +79,18 @@ public:
     GLXPixmap GetGLXPixmap();
 #endif
 
+    // Return true if cairo will take its slow path when this surface is used
+    // in a pattern with EXTEND_PAD.  As a workaround for XRender's RepeatPad
+    // not being implemented correctly on old X servers, cairo avoids XRender
+    // and instead reads back to perform EXTEND_PAD with pixman.  Cairo does
+    // this for servers older than xorg-server 1.7.
+    bool IsPadSlow() {
+        // The test here matches that for buggy_pad_reflect in
+        // _cairo_xlib_device_create.
+        return VendorRelease(mDisplay) >= 60700000 ||
+            VendorRelease(mDisplay) < 10699000;
+    }
+
 protected:
     // if TakePixmap() has been called on this
     bool mPixmapTaken;