Is your feature request related to a problem? Please describe.
Sometimes, you just want to check if something runs in software, be it lavapipe or WARP. We currently have no controls to expoes this, other than by enumerating the adapters.

Describe the solution you'd like
Perhaps, we could have a PowerPreference::Virtual or something like that?

**Describe alternatives you've cons

If the hello_triangle example is changed slightly to only draw the triangle once, and we change the color attachment's load_op to wgpu::LoadOp::Load instead of Clear, my expectation is that the result is the same, except that the green clear color is gone. However the result is that the red triangle only shows up every other frame, with blank frames in between.

Is this expected behavi