* Validate width and height inputs
- Fixes#157
* Add window size check to `imgui-winit` demo
- There is really nothing better that can be done in this case.
- The surface, buffer, and world sizes must all be non-zero.
- A zero-length surface would (previously) panic in `wgpu`.
- A zero-length buffer would panic with the new assertions.
- A zero-length world would cause a divide-by-zero panic when drawing.
cdcfe57868