Bug 1241161 - make Matrix4x4::ProjectTo2D normalize out perpective where possible. r=mattwoodrow

gfx/2d/Matrix.h

@@ -548,6 +548,24 @@ public:
     _33 = 1.0f;
     _43 = 0.0f;
     _34 = 0.0f;
+    // Some matrices, such as those derived from perspective transforms,
+    // can modify _44 from 1, while leaving the rest of the fourth column
+    // (_14, _24) at 0. In this case, after resetting the third row and
+    // third column above, the value of _44 functions only to scale the
+    // coordinate transform divide by W. The matrix can be converted to
+    // a true 2D matrix by normalizing out the scaling effect of _44 on
+    // the remaining components ahead of time.
+    if (_14 == 0.0f && _24 == 0.0f &&
+        _44 != 1.0f && _44 != 0.0f) {
+      Float scale = 1.0f / _44;
+      _11 *= scale;
+      _12 *= scale;
+      _21 *= scale;
+      _22 *= scale;
+      _41 *= scale;
+      _42 *= scale;
+      _44 = 1.0f;
+    }
     return *this;
   }