Improve portable builds

In portable builds, scrcpy-server.jar was supposed to be present in the
current directory, so in practice it worked only if scrcpy was launched
from its own directory.

Instead, find the absolute path of the executable and build a suitable
path to use scrcpy-server.jar from the same directory.

app/src/command.h

@@ -9,6 +9,7 @@
  // not needed here, but winsock2.h must never be included AFTER windows.h
 # include <winsock2.h>
 # include <windows.h>
+# define PATH_SEPARATOR '\\'
 # define PRIexitcode "lu"
 // <https://stackoverflow.com/a/44383330/1987178>
 # ifdef _WIN64
@@ -23,6 +24,7 @@
 #else
 
 # include <sys/types.h>
+# define PATH_SEPARATOR '/'
 # define PRIsizet "zu"
 # define PRIexitcode "d"
 # define PROCESS_NONE -1
@@ -76,4 +78,9 @@ adb_install(const char *serial, const char *local);
 bool
 process_check_success(process_t proc, const char *name);
 
+// return the absolute path of the executable (the scrcpy binary)
+// may be NULL on error; to be freed by SDL_free
+char *
+get_executable_path(void);
+
 #endif