Be explicit in our list-initialisation.

This caused https://github.com/facebookincubator/FBX2glTF/issues/218
in ways that I honestly don't fully understand, and I believe only under
Windows, with Release-level optimisation turned on.

It's possible we should drop all these initializer lists, and replace
them with simple and explicit initialising constructors.

Fixes #218.

src/raw/RawModel.hpp

@@ -29,9 +29,9 @@ enum RawVertexAttribute {
 };
 
 struct RawBlendVertex {
-  Vec3f position{};
-  Vec3f normal{};
-  Vec4f tangent{};
+  Vec3f position{0.0f};
+  Vec3f normal{0.0f};
+  Vec4f tangent{0.0f};
 
   bool operator==(const RawBlendVertex& other) const {
     return position == other.position && normal == other.normal && tangent == other.tangent;