Zero-copy decoding

[randy408]

Zero-copy decoding consists of decompressing scanlines directly to the output buffer, this would be limited to non-interlaced images with the following PNG format -> output format combinations:

• RGBA8->RGBA8
• RGBA16->RGBA16
• PNG -> PNG (New image format - PNG / RAW #24)
• GA16 -> RGBA8 with some tweaks

Format	Bytes per pixel
Indexed/Gray 1/2/4bit	1
Gray 8-bit	1
Gray 16-bit	2
Gray alpha 8-bit	2
Gray alpha 16-bit	4
RGB 8-bit	3
RGB 16-bit	6
RGBA 8-bit	4
RGBA 16-bit	8

This may reduce performance (or not work, depending on optimizations and platform) because the scanlines are not always aligned.

[randy408]

Saving this here for future reference: madler/zlib#149 (comment)