Bug 727681; correct gfx3DMatrix::CanDraw2D. r=Bas

gfx/thebes/gfx3DMatrix.cpp

@@ -714,8 +714,9 @@ gfx3DMatrix::Is2D(gfxMatrix* aMatrix) const
 bool
 gfx3DMatrix::CanDraw2D(gfxMatrix* aMatrix) const
 {
-  if (_14 != 0.0f || _24 != 0.0f ||
-      _34 != 0.0f || _44 != 1.0f) {
+  if (_14 != 0.0f ||
+      _24 != 0.0f ||
+      _44 != 1.0f) {
     return false;
   }
   if (aMatrix) {

gfx/thebes/gfx3DMatrix.h

@@ -114,7 +114,10 @@ public:
    * Returns true if the matrix can be reduced to a 2D affine transformation
    * (i.e. as obtained by From2D). If it is, optionally returns the 2D
    * matrix in aMatrix. This should only be used on matrices required for
-   * rendering, not for intermediate calculations.
+   * rendering, not for intermediate calculations. It is assumed that the 2D
+   * matrix will only be used for transforming objects on to the z=0 plane,
+   * therefore any z-component perspective is ignored. This means that if
+   * aMatrix is applied to objects with z != 0, the results may be incorrect.
    *
    * Since drawing is to a 2d plane, any 3d transform without perspective
    * can be reduced by dropping the z row and column.