Shapes can contain 3D effects like in the following example:

  <a:scene3d>
    <a:camera prst="isometricLeftDown" zoom="150000"/>
    <a:lightRig rig="threePt" dir="t"/>
  </a:scene3d>

This patch preserves the a:camera tag and its attributes using the
shape grab bag. It also adds a unit test for this case.

Change-Id: Ic6a78031d2e1fb84a2bacd97b5cc9c55d9dbaa95