[Bugfix] tonumber(ffi.C.wcslen())

[Akatmks]

We found that the current Yutils.lua will run into problems if character spacing is not set to 0. This is reproduced in sepro's and my test.

We had a look at several different versions of Yutils.lua, and we believe this tonumber() is the fix to the problem.

sepro told me to make this pull request. we will also inform arch1t3cht about this issue.

Steps to reproduce

1. Install zf.everythingShape from https://github.com/TypesettingTools/zeref-Aegisub-Scripts
2. Open style manager and set style character spacing to any value other than 0.0. For example, 1.0.
3. Create a line of subtitle and use the „Shape To Clip“ function inside zf.everythingShape on the subtitle.

Thank you.

[CoffeeFlux]

@Akatmks what value is returned when the spacing is non-zero? Doesn't wcslen always return a number - is this some issue with the ffi bindings? I'm a bit confused by this PR, or at least why that's the fix.

[Akatmks]

It seems like it is something with ffi. Before tonumber the return is a ffi cdata, and lua for seems to only accept lua number.