making spupng more modular

In constant.h I have added type of subtitle so that
encoder can know whether decoder can actually convert
its type or not. for example dvb subtitle have pixel block
not all decoder support pixel block as input(atleaset without ocr).

In spupng encoder i have added more function to make it moduler
Remove redundant code from 608_spupng

using function in spupng.c to write spu tags and comment
Support spupng output in dvbsub decoder
corrected initial time for dvb sub
removed printf and using mprint in dvb decoder
done indentation of dvbsubtitle module
making png with palette
Merge branch 'pr/n35_anshul1912'