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/sys/win/command.c

@@ -75,3 +75,18 @@ cmd_simple_wait(HANDLE handle, DWORD *exit_code) {
     }
     return !code;
 }
+
+char *
+get_executable_path(void) {
+    HMODULE hModule = GetModuleHandleW(NULL);
+    if (!hModule) {
+        return NULL;
+    }
+    WCHAR buf[MAX_PATH + 1]; // +1 for the null byte
+    int len = GetModuleFileNameW(hModule, buf, MAX_PATH);
+    if (!len) {
+        return NULL;
+    }
+    buf[len] = '\0';
+    return utf8_from_wide_char(buf);
+}