Cleanup: GPv3: Add static assets for sizes of classes

The C++ classes that wrap DNA structs should never store any data.
Runtime data needs to be stored on the runtime classes/structs
and anything else needs to be put into DNA.

This adds static asserts to make sure (at compile time) that these
classes have the same size as the DNA structs (i.e. they don't store
additional data).

source/blender/blenkernel/BKE_grease_pencil.hh

@@ -149,6 +149,7 @@ class Drawing : public ::GreasePencilDrawing {
   bool is_instanced() const;
   bool has_users() const;
 };
+static_assert(sizeof(Drawing) == sizeof(::GreasePencilDrawing));
 
 class DrawingReference : public ::GreasePencilDrawingReference {
  public:
@@ -156,6 +157,7 @@ class DrawingReference : public ::GreasePencilDrawingReference {
   DrawingReference(const DrawingReference &other);
   ~DrawingReference();
 };
+static_assert(sizeof(DrawingReference) == sizeof(::GreasePencilDrawingReference));
 
 /**
  * Copies the drawings from one array to another. Assumes that \a dst_drawings is allocated but not
@@ -294,6 +296,7 @@ class TreeNode : public ::GreasePencilLayerTreeNode {
    */
   int64_t depth() const;
 };
+static_assert(sizeof(TreeNode) == sizeof(::GreasePencilLayerTreeNode));
 
 /**
  * A layer mask stores a reference to a layer that will mask other layers.
@@ -305,6 +308,7 @@ class LayerMask : public ::GreasePencilLayerMask {
   LayerMask(const LayerMask &other);
   ~LayerMask();
 };
+static_assert(sizeof(LayerMask) == sizeof(::GreasePencilLayerMask));
 
 /**
  * Structure used to transform frames in a grease pencil layer.
@@ -576,6 +580,7 @@ class Layer : public ::GreasePencilLayer {
    */
   float4x4 parent_to_world(const Object &parent) const;
 };
+static_assert(sizeof(Layer) == sizeof(::GreasePencilLayer));
 
 class LayerGroupRuntime {
  public:
@@ -710,6 +715,7 @@ class LayerGroup : public ::GreasePencilLayerTreeGroup {
   void ensure_nodes_cache() const;
   void tag_nodes_cache_dirty() const;
 };
+static_assert(sizeof(LayerGroup) == sizeof(::GreasePencilLayerTreeGroup));
 
 inline void Drawing::add_user() const
 {