Add Brian's explanation for inheritance in C.

src/gallium/README.portability

@@ -24,6 +24,7 @@ headers in general, should stricly follow these guidelines to ensure
 * Don't use variable number of macro arguments. Use static inline functions
 instead.
 
+* Don't use C99 features.
 
 = Standard Library =
 
@@ -42,3 +43,67 @@ portable way.
 * Use the functions/macros in p_debug.h.
 
 * Don't include assert.h, call abort, printf, etc.
+
+
+= Code Style =
+
+== Inherantice in C ==
+
+The main thing we do is mimic inheritance by structure containment.
+
+Here's a silly made-up example:
+
+/* base class */
+struct buffer
+{
+  int size;
+  void (*validate)(struct buffer *buf);
+};
+
+/* sub-class of bufffer */
+struct texture_buffer
+{
+  struct buffer base;  /* the base class, MUST COME FIRST! */
+  int format;
+  int width, height;
+};
+
+
+Then, we'll typically have cast-wrapper functions to convert base-class 
+pointers to sub-class pointers where needed:
+
+static inline struct vertex_buffer *vertex_buffer(struct buffer *buf)
+{
+  return (struct vertex_buffer *) buf;
+}
+
+
+To create/init a sub-classed object:
+
+struct buffer *create_texture_buffer(int w, int h, int format)
+{
+  struct texture_buffer *t = malloc(sizeof(*t));
+  t->format = format;
+  t->width = w;
+  t->height = h;
+  t->base.size = w * h;
+  t->base.validate = tex_validate;
+  return &t->base;
+}
+
+Example sub-class method:
+
+void tex_validate(struct buffer *buf)
+{
+  struct texture_buffer *tb = texture_buffer(buf);
+  assert(tb->format);
+  assert(tb->width);
+  assert(tb->height);
+}
+
+
+Note that we typically do not use typedefs to make "class names"; we use
+'struct whatever' everywhere.
+
+Gallium's pipe_context and the subclassed psb_context, etc are prime examples 
+of this.  There's also many examples in Mesa and the Mesa state tracker.