Print info logs to stdout

All server logs were printed to stdout, while all client logs were
printed to stderr.

Instead, use stderr for warnings and errors, stdout for the others:
 - stdout: verbose, debug, info
 - stderr: warn, error

app/src/util/log.c

@@ -125,8 +125,30 @@ sc_av_log_callback(void *avcl, int level, const char *fmt, va_list vl) {
     free(local_fmt);
 }
 
+static const char *const sc_sdl_log_priority_names[SDL_NUM_LOG_PRIORITIES] = {
+    [SDL_LOG_PRIORITY_VERBOSE] = "VERBOSE",
+    [SDL_LOG_PRIORITY_DEBUG] = "DEBUG",
+    [SDL_LOG_PRIORITY_INFO] = "INFO",
+    [SDL_LOG_PRIORITY_WARN] = "WARN",
+    [SDL_LOG_PRIORITY_ERROR] = "ERROR",
+    [SDL_LOG_PRIORITY_CRITICAL] = "CRITICAL",
+};
+
+static void SDLCALL
+sc_sdl_log_print(void *userdata, int category, SDL_LogPriority priority,
+                 const char *message) {
+    (void) userdata;
+    (void) category;
+
+    FILE *out = priority < SDL_LOG_PRIORITY_WARN ? stdout : stderr;
+    assert(priority < SDL_NUM_LOG_PRIORITIES);
+    const char *prio_name = sc_sdl_log_priority_names[priority];
+    fprintf(out, "%s: %s\n", prio_name, message);
+}
+
 void
 sc_log_configure() {
+    SDL_LogSetOutputFunction(sc_sdl_log_print, NULL);
     // Redirect FFmpeg logs to SDL logs
     av_log_set_callback(sc_av_log_callback);
 }

server/src/main/java/com/genymobile/scrcpy/Ln.java

@@ -60,7 +60,7 @@ public final class Ln {
     public static void w(String message, Throwable throwable) {
         if (isEnabled(Level.WARN)) {
             Log.w(TAG, message, throwable);
-            System.out.print(PREFIX + "WARN: " + message + '\n');
+            System.err.print(PREFIX + "WARN: " + message + '\n');
             if (throwable != null) {
                 throwable.printStackTrace();
             }
@@ -74,7 +74,7 @@ public final class Ln {
     public static void e(String message, Throwable throwable) {
         if (isEnabled(Level.ERROR)) {
             Log.e(TAG, message, throwable);
-            System.out.print(PREFIX + "ERROR: " + message + "\n");
+            System.err.print(PREFIX + "ERROR: " + message + "\n");
             if (throwable != null) {
                 throwable.printStackTrace();
             }