Copy buffer-to-buffer error

**Description**
Error:
> copy would end up overruning the bounds of the destination buffer/texture

**Repro steps**
Code is in https://github.com/nical/lyon/blob/c921b427b6708665cf586302812170923fe302df/examples/wgpu_svg/src/main.rs#L478-L484

**Expected vs observed behavior**
Looks like it should be working.
