Bug 1479234 - Introduce a generic function to get an animation value on the compositor. r=boris,froydnj

On the compositor we store animation values in a hash table and the hash is
the compositor animation id which is a unique id for each property respectively.
So we can get the corresponding animation value for the given property.

In this patch there are lots of duplicated code, but they will be removed in the
next patch.

MozReview-Commit-ID: 7EboVcculcg

gfx/layers/AnimationHelper.cpp

@@ -85,6 +85,47 @@ CompositorAnimationStorage::GetAnimationTransform(const uint64_t& aId) const
   return Some(transform);
 }
 
+OMTAValue
+CompositorAnimationStorage::GetOMTAValue(const uint64_t& aId) const
+{
+  OMTAValue omtaValue = mozilla::null_t();
+  auto animatedValue = GetAnimatedValue(aId);
+  if (!animatedValue) {
+    return omtaValue;
+  }
+
+  switch (animatedValue->mType) {
+    case AnimatedValue::OPACITY:
+      omtaValue = animatedValue->mOpacity;
+      break;
+    case AnimatedValue::TRANSFORM: {
+      gfx::Matrix4x4 transform = animatedValue->mTransform.mFrameTransform;
+      const TransformData& data = animatedValue->mTransform.mData;
+      float scale = data.appUnitsPerDevPixel();
+      gfx::Point3D transformOrigin = data.transformOrigin();
+
+      // Undo the rebasing applied by
+      // nsDisplayTransform::GetResultingTransformMatrixInternal
+      transform.ChangeBasis(-transformOrigin);
+
+      // Convert to CSS pixels (this undoes the operations performed by
+      // nsStyleTransformMatrix::ProcessTranslatePart which is called from
+      // nsDisplayTransform::GetResultingTransformMatrix)
+      double devPerCss =
+        double(scale) / double(nsDeviceContext::AppUnitsPerCSSPixel());
+      transform._41 *= devPerCss;
+      transform._42 *= devPerCss;
+      transform._43 *= devPerCss;
+      omtaValue = transform;
+      break;
+    }
+    case AnimatedValue::NONE:
+      break;
+  }
+
+  return omtaValue;
+}
+
 void
 CompositorAnimationStorage::SetAnimatedValue(uint64_t aId,
                                              gfx::Matrix4x4&& aTransformInDevSpace,