update url on WebP format page

[hamish]

Hi,

there's a defunct url at ​http://www.gdal.org/frmt_webp.html

trunk/frmts/webp/frmt_webp.html:

- http://code.google.com/intl/fr/speed/webp/
+ https://developers.google.com/speed/webp/

also, it might be useful to mention in the help page that the maximum array size is limited to 16383 x 16383 (14 bit).

cheers, Hamish

[Even Rouault]

Thanks. Fixed in r27021