Separate multi-words filenames by '_'

Rename foobar.ext to foo_bar.ext.

<https://github.com/Genymobile/scrcpy/pull/226#discussion_r209454865>

app/src/lock_util.c

@@ -0,0 +1,33 @@
+#include <stdlib.h>
+#include <SDL2/SDL_mutex.h>
+
+#include "log.h"
+
+void mutex_lock(SDL_mutex *mutex) {
+    if (SDL_LockMutex(mutex)) {
+        LOGC("Could not lock mutex");
+        abort();
+    }
+}
+
+void mutex_unlock(SDL_mutex *mutex) {
+    if (SDL_UnlockMutex(mutex)) {
+        LOGC("Could not unlock mutex");
+        abort();
+    }
+}
+
+void cond_wait(SDL_cond *cond, SDL_mutex *mutex) {
+    if (SDL_CondWait(cond, mutex)) {
+        LOGC("Could not wait on condition");
+        abort();
+    }
+}
+
+void cond_signal(SDL_cond *cond) {
+    if (SDL_CondSignal(cond)) {
+        LOGC("Could not signal a condition");
+        abort();
+    }
+}
+