Spawned from #2062 (comment)

Our current API for declaring operations and read-only properties is not ideal:

    required (GPULoadOp or float) depthLoadValue;
    required GPUStoreOp depthStoreOp;
    boolean depthReadOnly = false;

Problems with it:

1. If my attachment doesn't have a depth component, I'm still forced to specify both load and store ops. The store op has to be "store" even though there can be no store performed. Same for stencil.
2. If the depth aspect is read-only, I'm still forced to specify the store operation as "store". Same for stencil.

Diagram for understanding the data and validation: