pngparse: improve parsing of the image

pngparse: improve parsing of the image

Everytime a buffer is being provided from baseparse, we are parsing all the data from the beginning.
But since we would have already parsed some of the data in the previous iterations,
it doesnt make much sense to keep parsing the same everytime.
Hence skipping the data which is already read in previous iterations to improve the parsing performance.

https://bugzilla.gnome.org/show_bug.cgi?id=740058