(enum OTF_ReaderFlag): New enum.

(enum OTF_ReaderFlag): New enum.
(OTF_TableInfo): Change the prototype of the member `reader'.
Caller changed.
(read_head_table, read_name_table, read_cmap_table)
(read_gdef_table): Adjusted for the change of OTF_TableInfo.
(read_feature_list): Seek the stream at first.
(read_gsub_gpos_table): Adjusted for the change of OTF_TableInfo.
Pay attention to the arg `flag'.
(read_gsub_table, read_gpos_table): Adjusted for the change of
OTF_TableInfo.
(OTF_get_table): Likewise.
(OTF_get_scripts, OTF_get_features, OTF_check_features): New
functions.