Fix handling of non-ASCII file paths on Windows for PNG read/write operations

[akchurinda]

Problem
PNGFiles could not reliably read or write PNG files on Windows when the file path contained non-ASCII characters, because the code used fopen instead of the wide-character Windows API. That meant paths with accented or otherwise non-ASCII names could fail even though they should be valid on Windows.

This issue is causing MakieOrg/Makie.jl#4749.

Changes
The core fix switches the file open calls in io.jl to _wfopen on Windows so Unicode file paths are handled correctly for both reading and writing. I also added a test in runtests.jl that saves a small PNG into a temporary Unicode-named directory and loads it back to verify round-trip correctness on all platforms.

[nhz2]

Some thing is broken with Windows CI as well:
Windows:

PNGFiles      |  922    922  1m08.0s

Ubuntu:

PNGFiles      | 1839   1839  1m06.0s

For some reason 900 tests are being skipped.

Edit: Fixed by #86

[akchurinda]

@asinghvi17 and @nhz2,
Thank you for taking your time on this PR. Happy the PR solved the other two.
When can we expect the new version to be published? It would solve a lot of downstream issues.

[nhz2]

It would be good to check if Makie still works on Windows in an environment with this version of PNGFiles.

Basically running something like:

using Pkg
Pkg.activate(;temp=true)
Pkg.develop("PNGFiles")
Pkg.develop("GLMakie")
Pkg.test("GLMakie")

I don't have access to Windows right now to test this.

[akchurinda]

@nhz2 I have asked a person because of whom I prepared this PR and It worked on his Windows machine (his path contains non-ASCII characters). I'll ask him to send me the output of GLMakie test and I'll post it here, unless someone does it first.

[nhz2]

I just tested on Windows as well, and it seems to be working.