Add a way to capture error messages.

[SpaceManiac]

For languages with error handling facilities, it would be useful to be able to capture messages output to sf::err() from CSFML. I suggest an API like:

sfErrorCapture* sfErrorCapture_start();
const char* sfErrorCapture_get(const sfErrorCapture* capture);
void sfErrorCapture_end(sfErrorCapture* capture);

I tried to implement this myself using std::ostringstream, but it seems like something is going odd in linking and each CSFML module has its own copy of the sf::err function, so placing these functions in the system module means that errors from other modules are not redirected.

[oprypin]

I support this.

[LaurentGomila]

Error handling will completely change in a future version of SFML, and as you pointed, there are multiple sf::err streams in CSFML, so there's no easy solution.

[zsbzsb]

It would be possible to add a function to each module to redirect the output into a single stream. Trouble with that approach is that if SFML is dynamically linked then sf::err will be set multiple times and it would be redundant. But that could be avoided with a few simple processor commands at compile time.