[lilliput] Add jpeg content length detection

[walruscow]

Walk jpeg segments to find EOI and report content length accurately Sadly this seems to require a linear scan of most of the content

a jpeg file consists of a sequence of segments

• segments are delimited by 0xFF, then a single non-zero byte for type. but ECS segment is not delimited by 0xFF
• EOI segment type is is 0xD9 while SOI is 0xD8
• most segments have 2 bytes for size following the type, but not the actual image data in entropy coded segments (ECS)
• in ECS, 0xFF may occur naturally and is marked as data instead of a new segment by adding 0x00 immediately after the 0xFF

Add a couple unit tests for jpeg data

• https://github.com/corkami/formats/blob/master/image/jpeg.md
• https://en.wikipedia.org/wiki/JPEG#Syntax_and_structure

[walruscow]

Nope, some segment is not entropy coded so we can't just search for 0xFF