Added thread-safe environment functions

Also marked the existing functions as unsafe, as they can cause crashes if used in multi-threaded applications.

As a bonus, since the new functions are hashtable based, hint environment lookups are much faster.

test/testtimer.c

@@ -124,7 +124,7 @@ int main(int argc, char *argv[])
         return 1;
     }
 
-    if (SDL_getenv("SDL_TESTS_QUICK") != NULL) {
+    if (SDL_GetEnvironmentVariable(SDL_GetEnvironment(), "SDL_TESTS_QUICK") != NULL) {
         SDL_Log("Not running slower tests");
         SDL_Quit();
         return 0;