WGPU: when copying to mip levels, copy correct size Bug: angleproject:389145696 Change-Id: I78a2a573de371c1439033ef4e43f84906b076d41 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6333237 Reviewed-by: Liza Burakova <liza@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Matthew Denton <mpdenton@chromium.org>