buffer: make _bind() return base pointer

We now make _cogl_buffer_bind return a base pointer for the bound buffer
which can be used with OpenGL. The pointer will be NULL for GPU based
buffers or may point to an malloc'd buffer. Since OpenGL expects an
offset instead of a pointer when dealing with buffer objects this means
we can handle fallback malloc buffers and GPU buffers in a consistent
way.

cogl/cogl-buffer-private.h

@@ -120,7 +120,7 @@ _cogl_buffer_initialize (CoglBuffer          *buffer,
 void
 _cogl_buffer_fini (CoglBuffer *buffer);
 
-void
+void *
 _cogl_buffer_bind (CoglBuffer *buffer,
                    CoglBufferBindTarget target);