Add a header that redefines g_array_index() to throw in a "void *" cast

Add a header that redefines g_array_index() to throw in a "void *" cast
to work around the "data" field of a GArray being a guint8 *, and
defines a g_array_data() macro to extract that field and cast it to void
*.

Use that header where needed.

svn path=/trunk/; revision=21627