Use UTF-8 for OpenAL string marshalling

[Morilli]

Marshal.PtrToStringAnsi uses the system codepage which is problematic especially on windows. OpenAL returns UTF-8 strings which should be parsed as UTF-8.

Using the following test code:

var api = ALContext.GetApi();
api.TryGetExtension<EnumerateAll>(null, out var enumerateAll);
var deviceEnumerable = enumerateAll.GetStringList(GetEnumerateAllContextStringList.AllDevicesSpecifier);
foreach (var deviceName in deviceEnumerable)
{
    Console.WriteLine(deviceName);
}

with a non-utf-8 codepage and a device named test ドットネット, I get:

before:
OpenAL Soft on test ãƒ%ッãƒ^ãƒ?ッãƒ^ (Realtek High Definition Audio)

after (with proper console encoding configured):
OpenAL Soft on test ドットネット (Realtek High Definition Audio)

[Perksey]

Thank you so much!