There is a use case of doing a render pass with no attachments. For example, when the pixel shader does some work with storage buffers, and we still want rasterization to happen, just without rendering per se.

Currently what happens is - we don't know the rasterization area when the render pass is started, and our implementation panics. It has no way to know this today.

Possible solutions:

1. say this is an unsupported use case. Users could technically have a dummy attachment and set up the blend equations to always fail...
2. have an optional member of RenderPassDescriptor with the rasterization area. When not set, it's automatically derived from the attachments, which are expected to be of the same size.