Update mViewProjectionMatrix after modiying mViewMatrix

I added an ArcballCamera to google#428 and noticed that it wasn't working as
expected.

ArcballCamera wasn't updating mViewProjectionMatrix after modifying
mViewMatrix. This was causing the ArcballCamera to not work correctly
when combined with the classes introduced in google#426.

src/ppx/camera.cpp

@@ -301,6 +301,7 @@ void ArcballCamera::UpdateCamera()
 {
     mViewMatrix        = mTranslationMatrix * glm::mat4_cast(mRotationQuat) * mCenterTranslationMatrix;
     mInverseViewMatrix = glm::inverse(mViewMatrix);
+    mViewProjectionMatrix = mProjectionMatrix * mViewMatrix;
 
     // Transform the view space origin into world space for eye position
     mEyePosition = mInverseViewMatrix * float4(0, 0, 0, 1);