Bug 573687 - Clamp radial gradients with small radii correctly. r=jwatt

layout/reftests/svg/radialGradient-basic-03-ref.svg

@@ -25,10 +25,15 @@
             <stop offset="0" stop-color="lime" />
             <stop offset="1" stop-color="red"/>
         </radialGradient>
+        <radialGradient id="gradient5" r="0.2" fx="0.308">
+          <stop offset="0" stop-color="lime" />
+          <stop offset="1" stop-color="red"/>
+        </radialGradient>
     </defs>
 
     <circle cx="100" cy="100" r="50" fill="url(#gradient1)"/>
     <circle cx="100" cy="200" r="50" fill="url(#gradient2)"/>
     <circle cx="200" cy="100" r="50" fill="url(#gradient3)"/>
     <circle cx="200" cy="200" r="50" fill="url(#gradient4)"/>
+    <circle cx="300" cy="100" r="50" fill="url(#gradient5)"/>
 </svg>

layout/reftests/svg/radialGradient-basic-03.svg

@@ -25,10 +25,15 @@
             <stop offset="0" stop-color="lime" />
             <stop offset="1" stop-color="red"/>
         </radialGradient>
+        <radialGradient id="gradient5" r="0.2" fx="0.3">
+          <stop offset="0" stop-color="lime" />
+          <stop offset="1" stop-color="red"/>
+        </radialGradient>
     </defs>
 
     <circle cx="100" cy="100" r="50" fill="url(#gradient1)"/>
     <circle cx="100" cy="200" r="50" fill="url(#gradient2)"/>
     <circle cx="200" cy="100" r="50" fill="url(#gradient3)"/>
     <circle cx="200" cy="200" r="50" fill="url(#gradient4)"/>
+    <circle cx="300" cy="100" r="50" fill="url(#gradient5)"/>
 </svg>

layout/svg/base/src/nsSVGGradientFrame.cpp

@@ -587,14 +587,11 @@ nsSVGRadialGradientFrame::CreateGradient()
     // The focal point (fFx and fFy) must be clamped to be *inside* - not on -
     // the circumference of the gradient or we'll get rendering anomalies. We
     // calculate the distance from the focal point to the gradient center and
-    // make sure it is *less* than the gradient radius. 0.99 is used as the
-    // factor of the radius because it's close enough to 1 that we won't get a
-    // fringe at the edge of the gradient if we clamp, but not so close to 1
-    // that rounding error will give us the same results as using fR itself.
-    // Also note that .99 < 255/256/2 which is the limit of the fractional part
-    // of cairo's 24.8 fixed point representation divided by 2 to ensure that
-    // we get different cairo fractions
-    double dMax = 0.99 * r;
+    // make sure it is *less* than the gradient radius.
+    // 1/128 is the limit of the fractional part of cairo's 24.8 fixed point
+    // representation divided by 2 to ensure that we get different cairo
+    // fractions
+    double dMax = NS_MAX(0.0, r - 1.0/128);
     float dx = fx - cx;
     float dy = fy - cy;
     double d = sqrt((dx * dx) + (dy * dy));