codec/mjpeg: modify lex.go Lex function to work with JPEGs containing thumbnails

Currently the lexer uses the SOI and EOI markers i.e. 0xffd8 and 0xffd9 to indicate the start and end of image, however, some JPEGs depending on the source, will encode thumbnails also inside a JPEG, so the current lexer may think it’s got a full JPEG at the wrong time. Let’s adapt the current lexer to consider the possibility of thumbnails.