Use platform::request() only to obtain data

[kkaefer]

Currently, the platform::request() is also responsible for generating the URL (which should be part of the stylesheet), and for parsing the data tile. Instead, we should make this a generic HTTP request function and do all the processing directly in the renderer. This involves creating threads for parsing the data tiles in the renderer itself too.

The purpose of this is to have better control on parsing and the associated threads, as well as being able to use platform::request() to request JSON and sprite images too.

[kkaefer]

Completed in c7b012b