Add cli option to set the verbosity level

The verbosity was set either to info (in release mode) or debug (in
debug mode).

Add a command-line argument to change it, so that users can enable debug
logs using the release:

    scrcpy -Vdebug

app/src/util/log.h

@@ -3,6 +3,13 @@
 
 #include <SDL2/SDL_log.h>
 
+enum sc_log_level {
+    SC_LOG_LEVEL_DEBUG,
+    SC_LOG_LEVEL_INFO,
+    SC_LOG_LEVEL_WARN,
+    SC_LOG_LEVEL_ERROR,
+};
+
 #define LOGV(...) SDL_LogVerbose(SDL_LOG_CATEGORY_APPLICATION, __VA_ARGS__)
 #define LOGD(...) SDL_LogDebug(SDL_LOG_CATEGORY_APPLICATION, __VA_ARGS__)
 #define LOGI(...) SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, __VA_ARGS__)