[Netsurf-develop] Image support

[James B. <bu...@us...>]

Please try the latest build at http://netsurf.strcprstskrzkrk.co.uk/.
This adds support for images in web pages, JPEGs only at the moment. There
are some problems with it at the moment:

- if a page has the same image twice, it is downloaded twice
- alt text isn't shown before the image loads or if it fails
- only JPEGs supported by the RISC OS renderer work

James