gdigrab: allow capturing a window by its handle

x11grab can capture windows by their ID, but gdigrab can only capture
windows by their names, internally calling FindWindowW to lookup its
handle.

This patch simply allows the user to specify a window handle directly.
Signed-off-by: Lena <lena@nihil.gay>

Changelog

@@ -9,6 +9,7 @@ version <next>:
 - aap filter
 - demuxing, decoding, filtering, encoding, and muxing in the
   ffmpeg CLI now all run in parallel
+- enable gdigrab device to grab a window using the hwnd=HANDLER syntax
 
 version 6.1:
 - libaribcaption decoder

doc/indevs.texi

@@ -722,7 +722,7 @@ Win32 GDI-based screen capture device.
 
 This device allows you to capture a region of the display on Windows.
 
-There are two options for the input filename:
+Amongst options for the imput filenames are such elements as:
 @example
 desktop
 @end example
@@ -730,9 +730,13 @@ or
 @example
 title=@var{window_title}
 @end example
+or
+@example
+hwnd=@var{window_hwnd}
+@end example
 
 The first option will capture the entire desktop, or a fixed region of the
-desktop. The second option will instead capture the contents of a single
+desktop. The second and third options will instead capture the contents of a single
 window, regardless of its position on the screen.
 
 For example, to grab the entire desktop using @command{ffmpeg}:

libavdevice/gdigrab.c

@@ -273,9 +273,22 @@ gdigrab_read_header(AVFormatContext *s1)
         }
     } else if (!strcmp(filename, "desktop")) {
         hwnd = NULL;
+    } else if (!strncmp(filename, "hwnd=", 5)) {
+        name = filename + 5;
+        char *p;
+
+        hwnd = strtol(name, &p, 0);
+
+        if (p == NULL || p == name || p[0] == '\0')
+        {
+            av_log(s1, AV_LOG_ERROR,
+                   "Invalid window handle '%s', must be a valid integer.\n", name);
+            ret = AVERROR(EINVAL);
+            goto error;
+        }
     } else {
         av_log(s1, AV_LOG_ERROR,
-               "Please use \"desktop\" or \"title=<windowname>\" to specify your target.\n");
+               "Please use \"desktop\", \"title=<windowname>\" or \"hwnd=<hwnd>\" to specify your target.\n");
         ret = AVERROR(EIO);
         goto error;
     }