Make the logging system static

No initialization is needed anymore and we can use the logging functions
safely at any point of the code (threading is not handled though, so you
might get multiplexed messages if you log from two threads
simultaneously).

bin/render.cpp

@@ -5,15 +5,10 @@
 
 #include <mbgl/platform/default/headless_view.hpp>
 #include <mbgl/platform/default/headless_display.hpp>
+#include <mbgl/platform/log.hpp>
 #include <mbgl/storage/default_file_source.hpp>
 #include <mbgl/storage/default/sqlite_cache.hpp>
 
-#if __APPLE__
-#include <mbgl/platform/darwin/log_nslog.hpp>
-#else
-#include <mbgl/platform/default/log_stderr.hpp>
-#endif
-
 #pragma GCC diagnostic push
 #ifndef __clang__
 #pragma GCC diagnostic ignored "-Wunused-local-typedefs"
@@ -71,13 +66,6 @@ int main(int argc, char *argv[]) {
 
     using namespace mbgl;
 
-
-#if __APPLE__
-    Log::Set<NSLogBackend>();
-#else
-    Log::Set<StderrLogBackend>();
-#endif
-
     mbgl::SQLiteCache cache(cache_file);
     mbgl::DefaultFileSource fileSource(&cache);