unhandled overflow in validate_texture_copy_range

[jimblandy]

wgpu_core::command::transfer::validate_texture_copy_range checks that each axis of the orthotope being copied falls within the texture's boundaries with code like this:

let x_copy_max = texture_copy_view.origin.x + copy_size.width;
if x_copy_max > extent.width {
    return Err(...)
}

If the sum of texture_copy_view.origin.x and copy_size.width overflow in a release build, their sum may fall within extent.width even though the actual range is bad.