Fixed issue that prevented the two arguments from being used together

[KaleidonKep99]

I love this program and I use it daily to fix Windows 10+'s horrible issue where it just forgets to create previews for PNGs bigger than 1440p.

I wanted to set up a task that would periodically refresh the previews of a folder of mine, which gets new pictures I take in VRChat every few days or so.

I wasn't able to use the two arguments (-r and -s) together, because of how the program handled the arguments internally, so I fixed the code a bit.

I also took the liberty to update the project to .NET 4.8.

[bruhov]

Hi! Thank you for your contribution. But you already can use the two arguments together:
WinThumbsPreloader.exe -rs "path"

[VL4DST3R]

-rs

To be honest i had no idea that's how you were supposed to use multiple arguments here, i always expected -r -s to work like in any other program. Maybe consider merging his changes since its a lot more intuitive?