Take screenshot with XGetImage on x11 to workaround nvidia driver (nvfbc) limitation that only allows one nvfbc session at a time

src/capture/portal.c

@@ -8,10 +8,9 @@
 #include <stdlib.h>
 #include <stdio.h>
 #include <unistd.h>
+#include <limits.h>
 #include <assert.h>
 
-#include <libavcodec/avcodec.h>
-
 typedef struct {
     gsr_capture_portal_params params;