XLib: change opacity of child window (MPV)

Question

I have this code in C:

#include <X11/X.h>
#include <X11/Xlib.h>
#include <mpv/client.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// Handle MPV errors
void checkMPV(int status) {
  if (status < 0) {
    printf("MPV error: %s\n", mpv_error_string(status));
    exit(1);
  }
}

int main(void) {
  Display *display = XOpenDisplay(NULL);
  if (display == NULL) {
    fprintf(stderr, "Cannot open display\n");
    exit(1);
  }

  // Create X11 window
  int screen = DefaultScreen(display);
  Window window = XCreateSimpleWindow(
      display, RootWindow(display, screen), 10, 10, 100, 100, 1,
      BlackPixel(display, screen), WhitePixel(display, screen));
  XSelectInput(display, window, ExposureMask);
  XMapWindow(display, window);

  // Create MPV context
  mpv_handle *ctx = mpv_create();
  if (!ctx) {
    printf("failed to create context\n");
    return 1;
  }

  // bind MPV to master window
  char wid[64];
  sprintf(wid, "0x%lx", window);
  checkMPV(mpv_set_option_string(ctx, "wid", wid));

  // configure MPV
  checkMPV(mpv_set_option_string(ctx, "keepaspect", "no"));
  checkMPV(mpv_set_option_string(ctx, "x11-bypass-compositor", "no"));
  checkMPV(mpv_set_option_string(ctx, "gapless-audio", "yes"));
  checkMPV(mpv_set_option_string(ctx, "aid", "no"));
  checkMPV(mpv_set_option_string(ctx, "vo", "xv"));
  checkMPV(mpv_set_option_string(ctx, "hwdec", "auto"));
  checkMPV(mpv_set_option_string(ctx, "input-default-bindings", "no"));
  checkMPV(mpv_set_option_string(ctx, "input-vo-keyboard", "no"));

  int flag = 0;
  checkMPV(mpv_set_option(ctx, "osc", MPV_FORMAT_FLAG, &flag));

  // Initialize MPV
  checkMPV(mpv_initialize(ctx));

  // Play video
  const char *cmd[] = {"loadfile", "video.mp4", NULL};
  checkMPV(mpv_command(ctx, cmd));

  // Draw rectangle in loop
  XEvent event;
  while (1) {
    XNextEvent(display, &event);
    XClearWindow(display, window);
    XFillRectangle(display, window, DefaultGC(display, screen), 10, 10, 300,
                   300);
  }

  // Cleanup
  mpv_terminate_destroy(ctx);
  XCloseDisplay(display);
  return 0;
}

It attaches MPV window to X11 window. It looks like this (just a video frame):

The question is, can I make MPV window semi-transparent (for example, by 70%), so rectangle drawed by XFillRectangle(display, window, DefaultGC(display, screen), 10, 10, 300, 300); will be kinda visible? It would look like this:

Is it possible without using things like opengl?