Fix shader loading on Linux (#26)

Fixes two issues:
- Load the Linux shader assetbundle instead of the OSX version on Linux
- The new assetbundle from #24 contain files other than the shaders -
  both intended (the new .cginc files), and a few unintended ones (in a
  folder named b9, there are .obj and .mat files). For the former, fix
  the exception that occured when trying to load those files as shaders.
  For the latter, the assetbundles should be recompiled without those
  files.

src/Utilities/KKGraphics.cs

@@ -67,7 +67,7 @@ internal static void LoadShaderBundles ()
                     bundleFileName = "kkshaders.osx";
                     break;
                 case RuntimePlatform.LinuxPlayer:
-                    bundleFileName = "kkshaders.osx";
+                    bundleFileName = "kkshaders.linux";
                     break;
                 default:
                     bundleFileName = "kkshaders.windows";
@@ -106,8 +106,10 @@ internal static void LoadShaderBundles ()
         private static void LoadAndRegisterShader(AssetBundle bundle , string shaderName)
         {
             Shader newShader = bundle.LoadAsset<Shader>(shaderName);
+            if (newShader == null) { return; } // This file is not a shader; ignore it.
+
             GameObject.DontDestroyOnLoad(newShader);
-            if (newShader == null || !newShader.isSupported)
+            if (!newShader.isSupported)
             {
                 Log.Error("could not load shader: " + shaderName + " from: " + bundle.name);
             }