imwrite should accept lazy slice

[9il]

• lazy image / image with sride!1 != 1 -> buffering (4 KB buffer) -> writer stream
• image with srride!1 == 1 -> writer stream
• image with ideal strides -> writer stream (single call).

For example void write_png(Writer stream, long w, long h, in ubyte[] data, long tgt_chans = 0) from image formats can be used.