Use pkgconfig for bulletphysics

Change-Id: I684fe8b5abb03a44aed49819d3bad96cc11188cf
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Assimp external dependency

The AssimpParser is now a SceneParser plugin that is build if config tests for
assimp succeed.

Change-Id: I0e3f6a6ce2a43298b71b0261084f6c79521f214d
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
RenderTarget backend class

Change-Id: I914267b6efb9e99976172a795be5d2f17b2b3230
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
RenderAttachment backend class

Change-Id: I9a5dda2f1489938df08f2b881580b7a613f80652
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>