Bug 615620

Originally reported by: igorm@users.sourceforge.net
psf_write_truetype_data writes TT font with incorrect 
checksums for most tables. It is small importance 
because most TT interpreters ignore them. Perhaps a 
complete fix should generate TT data in 2 passes : the 
first pass computes the checksums, the second one 
really writes data. So complex due to the output stream 
isn't positionable (likely it's a FlateEncode filter). 

I'd suggest to implement a special encoding filter for 
checksums, and execute the body of 
psf_write_truetype_data twice : first with the checksum 
filter, second with the real output stream. After a TT 
table is completed, its checksum to be taken from the 
filter and to be put into the 'tables' array.