Add depth and stencil tests. This is currently only implemented for the
OpenGL API. HelloDepthStencil is a test using the depth and stencil
buffers to do reflections. Currently clearing / stencil clearing is not
working properly.
* First API design (many features missing, including input attachments)
* Metal implementation (no OpenGL yet)
* Render-to-texture demo (a little broken until we have depth buffers)
* Update examples to use render passes