Add option to change the push target

A drag & drop always pushed the file to /sdcard/.

Add an option to customize the target directory.

Fixes <https://github.com/Genymobile/scrcpy/issues/659>

app/src/scrcpy.h

@@ -10,6 +10,7 @@ struct scrcpy_options {
     const char *crop;
     const char *record_filename;
     const char *window_title;
+    const char *push_target;
     enum recorder_format record_format;
     uint16_t port;
     uint16_t max_size;