Description

We currently validate that all vertex buffers are within range, but not that they are existent.

Repro steps

Replace https://github.com/sotrh/learn-wgpu/blob/master/code/beginner/tutorial7-instancing/src/main.rs#L587 with index 0 so vertex buffer 1 isn't set.

Expected vs observed behavior

We need to error in this case.

Additional Information

We shou

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