Demo might need some fixes and update

[XiaochaoYan]

Compiling on a .Net8 and Win11 PC.

Some of the errors.

1. on file "BasicDemo.cs" line 74
   CS0019 Operator '*' cannot be applied to operands of type 'Matrix' and 'Matrix4' BasicDemo C:\Users\XXXXX\Desktop\BulletSharp-master\demos\OpenTK\BasicDemo\BasicDemo.cs 77

2. on file "Physics.cs" line 32
   CS0029 Cannot implicitly convert type 'OpenTK.Vector3' to 'BulletSharp.Math.Vector3' BasicDemo C:\Users\XXXXX\Desktop\BulletSharp-master\demos\OpenTK\BasicDemo\Physics.cs 32

Basically some data type conversion/parsing issues.