Invoke grim when taking screenshot

Backends should provide their own UI. As I don't want to copy-paste grim and
slurp here, let's just exec them. This should be made configurable in the
future.

screenshot.c

@@ -1,11 +1,41 @@
 #include <errno.h>
+#include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <sys/wait.h>
+#include <unistd.h>
 #include "xdpw.h"
 
 static const char object_path[] = "/org/freedesktop/portal/desktop";
 static const char interface_name[] = "org.freedesktop.impl.portal.Screenshot";
 
+static bool exec_screenshooter(const char *path) {
+	pid_t pid = fork();
+	if (pid < 0) {
+		perror("fork");
+		return false;
+	} else if (pid == 0) {
+		char *const argv[] = {
+			"grim",
+			"--",
+			(char *)path,
+			NULL,
+		};
+		execvp("grim", argv);
+
+		perror("execvp");
+		exit(127);
+	}
+
+	int stat;
+	if (waitpid(pid, &stat, 0) < 0) {
+		perror("waitpid");
+		return false;
+	}
+
+	return stat == 0;
+}
+
 static int method_screenshot(sd_bus_message *msg, void *data,
 		sd_bus_error *ret_error) {
 	int ret = 0;
@@ -55,8 +85,17 @@ static int method_screenshot(sd_bus_message *msg, void *data,
 		return ret;
 	}
 
-	// TODO: take an actual screenshot
+	// TODO: choose a better path
-	ret = sd_bus_message_append_basic(reply, 's', "file:///tmp/out.png");
+	const char path[] = "/tmp/out.png";
+	if (!exec_screenshooter(path)) {
+		return -1;
+	}
+
+	const char uri_prefix[] = "file://";
+	char uri[strlen(path) + strlen(uri_prefix) + 1];
+	snprintf(uri, sizeof(uri), "%s%s", uri_prefix, path);
+
+	ret = sd_bus_message_append_basic(reply, 's', uri);
 	if (ret < 0) {
 		return ret;
 	}