avcodec/pngdec: create a function to decode IDAT chunk.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/pngdec: create a function to decode PLTE chunk.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/pngdec: create a function to decode tRNS chunk.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/pngdec: fix some indentation/whitespaces

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>