Fix build error on older c# versions.

ReFuel.StbImage.csproj

@@ -13,7 +13,7 @@
     <!-- Nuget Properties. -->
     <GeneratePackageOnBuild>True</GeneratePackageOnBuild>
     <PackageId>ReFuel.StbImage</PackageId>
-    <Version>1.1.0-rc0</Version>
+    <Version>2.0-rc</Version>
     <Authors>STBI Authors, H. Utku Maden</Authors>
     <Description>
       A C# wrapper for the ubiquitous stb_image.h and stb_image_write.h library.
@@ -21,6 +21,13 @@
     <PackageReadmeFile>README.md</PackageReadmeFile>
     <PackageLicenseFile>LICENSE.md</PackageLicenseFile>
     <PackageIcon>images\icon.png</PackageIcon>
+    <PackageProjectUrl>https://refuel.mixedup.dev/docs/ReFuel.StbImage.html</PackageProjectUrl>
+    <RepositoryUrl>https://git.mixedup.dev/ReFuel/ReFuel.StbImage</RepositoryUrl>
+    <RepositoryType>git</RepositoryType>
+    <PackageTags>stb; stb_image; stbi; image; load; save; read; write</PackageTags>
+    <PackageReleaseNotes># 2.0.0
+* Exposed stbi_image_write.h
+* Moved native function calls to ReFuel.Stb.Native</PackageReleaseNotes>
   </PropertyGroup>
 
   <!--
@@ -30,8 +37,8 @@
     that determines a lot of load paths for projects.
   -->
   <ItemGroup>
-    <Content Include="*.md" Pack="true" PackagePath="/"/>
-    <None Include="rf_stbimage.png" Pack="true" PackagePath="images\icon.png"/>
+    <Content Include="*.md" Pack="true" PackagePath="/" />
+    <None Include="rf_stbimage.png" Pack="true" PackagePath="images\icon.png" />
     <Content Include="runtimes/linux-arm/native/*.so">
       <PackagePath>runtimes/linux-arm/native/</PackagePath>
       <Pack>true</Pack>

StbiStreamWrapper.cs

@@ -98,13 +98,13 @@ namespace ReFuel.Stb
     /// <summary>
     /// An easy to use stream wrapper for STBI image write functions.
     /// </summary>
-    /// <remarks>Keep struct alive for the duration of the write operation.</remarks>
-    public struct StbiWriteStreamWrapper
+    /// <remarks>Keep object alive for the duration of the write operation.</remarks>
+    public class StbiWriteStreamWrapper
     {
         private readonly Stream _stream;
         private readonly StbiWriteProc _cb;
 
-        public IntPtr Callback => Marshal.GetFunctionPointerForDelegate(_cb);
+        public IntPtr Callback { get; };
 
         public StbiWriteStreamWrapper(Stream stream)
         {
@@ -113,6 +113,7 @@ namespace ReFuel.Stb
             {
                 _cb = WriteCb;
             }
+            Callback = Marshal.GetFunctionPointerForDelegate(_cb);
         }
 
         private unsafe void WriteCb(void *context, void *data, int size)