ttf: Use SDL_GetError for initialization errors

Previously, the code use the last OS error. However, not all errors
returned by `TTF_Init` are OS errors, and the expected behaviour is that
the caller instead use `SDL_GetError` when an error is encountered.

To limit breakage for existing application the error is still returned
as an `io::Error`, albeit one with `io::ErrorKind::Other`. It is
worth noting that this does lead to an observable difference in
behaviour for callers – the error has a different kind – although any
code that is inspecting this was already relying on an unreliable source
of data.

This fixes issue #1347.

src/sdl2/ttf/context.rs

@@ -124,7 +124,10 @@ pub fn init() -> Result<Sdl2TtfContext, InitError> {
         } else if ttf::TTF_Init() == 0 {
             Ok(Sdl2TtfContext)
         } else {
-            Err(InitError::InitializationError(io::Error::last_os_error()))
+            Err(InitError::InitializationError(io::Error::new(
+                io::ErrorKind::Other,
+                get_error(),
+            )))
         }
     }
 }