MPEG TS: don't exit on MPEG-1/2 errors
Build system: don't warn about missing field initializers
memory_c: convenience function for pointing to an unmodifiable string
mkvmerge: refactor text subs packetizer not to rely on 0-termination in packet
MPEG TS reader: remove superfluous code
MPEG TS reader: refactor PMT descriptor parsing to smaller functions
MPEG TS: implemented teletext subtitle support

Implements #773.