Migrate recent HLLib work to BOS

It's better suited for the model of BOS and not an update to HLLibSharp

BurnOutSharp.Models/BSP/TextureHeader.cs

@@ -0,0 +1,16 @@
+namespace BurnOutSharp.Models.BSP
+{
+    /// <see href="https://github.com/RavuAlHemio/hllib/blob/master/HLLib/BSPFile.h"/>
+    public sealed class TextureHeader
+    {
+        /// <summary>
+        /// Texture count
+        /// </summary>
+        public uint TextureCount;
+
+        /// <summary>
+        /// Offsets
+        /// </summary>
+        public uint[] Offsets;
+    }
+}