Introduce render passes

[Kangz]

NXT uses a default framebuffer style of rendering as a placeholder for render passes. For automated testing we need to read-back what was rendered so we need to add render passes.

TODO (@kainino0x):

• First design Render Passes #14
• First implementation (Metal backend) Render Passes #14
• Render-to-texture demo Render Passes #14
• Update other demos Render Passes #14
• Update glTFViewer Update glTFViewer for render passes and depth stencil state #39
• Check fragment shader output types matches render pass metadata
• Testing?
• OpenGL backend OpenGL backend: implement render passes #73
• Any TODOs I added in Render Passes #14 - especially the backbuffer hack (blocked on WSI)
• load/store operations

[kainino0x]

First implementation is done in #14.

In particular:

• 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