FS#11399 by me: fix r26998 for text_viewer

Restore the old behaviour:

- preferences must be read-write for tv_preferences.c , read-only for
  all other modules -> use pointer to const struct
- init functions must get the plugin buffer + its size as arguments for
  easily adding new functions -> use pointer to buffer pointer and size
  to make allocation easier
- preferences meaning is private to each file and must not be known by
  tv_preferences.c -> move tv_check_header_and_footer() back in
  tv_window.c; also avoid chaining 3 times the callbacks by calling
  tv_set_preferences() only once if more than one preference needs
  changing

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@27089 a1c6a512-1295-4272-9138-f99709370657

apps/plugins/text_viewer/tv_text_processor.c

@@ -544,11 +544,16 @@ int tv_create_formed_text(const unsigned char *src, ssize_t bufsize,
     return size;
 }
 
-void tv_init_text_processor(void)
+bool tv_init_text_processor(unsigned char **buf, size_t *size)
 {
+    /* unused : no need for dynamic buffer yet */
+    (void)buf;
+    (void)size;
+
     text_type = TV_TEXT_UNKNOWN;
     expand_extra_line = false;
     is_break_line = false;
+    return true;
 }
 
 void tv_set_creation_conditions(int blocks, int width)