Fix blendToIdentity case in PerspectiveTransformOperation::blend

Source/platform/transforms/PerspectiveTransformOperation.cpp

Index: Source/platform/transforms/PerspectiveTransformOperation.cpp
===================================================================
--- Source/platform/transforms/PerspectiveTransformOperation.cpp	(revision 160782)
+++ Source/platform/transforms/PerspectiveTransformOperation.cpp	(working copy)
@@ -40,7 +40,7 @@
 
     if (blendToIdentity) {
         double p = floatValueForLength(m_p, 1);
-        p = WebCore::blend(p, 1.0, progress); // FIXME: this seems wrong. https://bugs.webkit.org/show_bug.cgi?id=52700
+        p = 1.0 / WebCore::blend(1.0 / p, 0.0, progress);

[Julien - ping for review, 2013/12/05 01:18:45]

This formula is overly complicated and it took me a while to understand what was
going on. It can be simplified to:

p = p / (1 - progress)

This uncovers an issue in the formula: if progress == 1, we are going to do a
division by 0.

         return PerspectiveTransformOperation::create(Length(clampToPositiveInteger(p), Fixed));
     }