WIP: Just playing with ffigen

Many structs end up empty, because dart:ffi does not yet support nested
structs: dart-lang/sdk#37271

```
Running in Directory: '/home/jpnurmi/Projects/dart_assimp'
[WARNING]: Prefer adding Key 'description' to your config.
Input Headers: [/usr/include/assimp/cexport.h, /usr/include/assimp/cfileio.h, /usr/include/assimp/cimport.h, /usr/include/assimp/postprocess.h, /usr/include/assimp/scene.h, /usr/include/assimp/version.h]
[WARNING]: Removed All Struct Members from aiRay(aiRay), Nested Structures not supported.
[WARNING]: Removed All Struct Members from aiExportDataBlob(aiExportDataBlob), Nested Structures not supported.
[WARNING]: Skipped Function 'aiGetPredefinedLogStream', struct pass/return by value not supported.
[WARNING]: Removed All Struct Members from aiTexture(aiTexture), Nested Structures not supported.
[WARNING]: Removed All Struct Members from aiAABB(aiAABB), Nested Structures not supported.
[WARNING]: Removed All Struct Members from aiBone(aiBone), Nested Structures not supported.
[WARNING]: Removed All Struct Members from aiAnimMesh(aiAnimMesh), Nested Structures not supported.
[WARNING]: Removed All Struct Members from aiMesh(aiMesh), Nested Structures not supported.
[WARNING]: Removed All Struct Members from aiLight(aiLight), Nested Structures not supported.
[WARNING]: Removed All Struct Members from aiCamera(aiCamera), Nested Structures not supported.
[WARNING]: Removed All Struct Members from aiUVTransform(aiUVTransform), Nested Structures not supported.
[WARNING]: Removed All Struct Members from aiMaterialProperty(aiMaterialProperty), Nested Structures not supported.
[WARNING]: Removed All Struct Members from aiVectorKey(aiVectorKey), Nested Structures not supported.
[WARNING]: Removed All Struct Members from aiQuatKey(aiQuatKey), Nested Structures not supported.
[WARNING]: Removed All Struct Members from aiNodeAnim(aiNodeAnim), Nested Structures not supported.
[WARNING]: Removed All Struct Members from aiMeshAnim(aiMeshAnim), Nested Structures not supported.
[WARNING]: Removed All Struct Members from aiMeshMorphAnim(aiMeshMorphAnim), Nested Structures not supported.
[WARNING]: Removed All Struct Members from aiAnimation(aiAnimation), Nested Structures not supported.
[WARNING]: Removed All Struct Members from aiNode(aiNode), Nested Structures not supported.
Finished, Bindings generated in /home/jpnurmi/Projects/dart_assimp/lib/src/bindings.dart
```

ffigen.yaml

@@ -0,0 +1,25 @@
+name: 'LibAssimp'
+output: 'lib/src/bindings.dart'
+headers:
+  entry-points:
+    - '/usr/include/assimp/cexport.h'
+    - '/usr/include/assimp/cfileio.h'
+    - '/usr/include/assimp/cimport.h'
+    - '/usr/include/assimp/postprocess.h'
+    - '/usr/include/assimp/scene.h'
+    - '/usr/include/assimp/version.h'
+comments: false
+functions:
+  include:
+    - 'ai.*'
+structs:
+  include:
+    - 'ai.*'
+enums:
+  include:
+    - 'ai.*'
+macros:
+  include:
+    - 'AI.*'
+array-workaround: true
+unnamed-enums: false