Add Win32 GDI-based screen grabbing

Based on original code by Christophe Gisquet in 2010, updated to work
with current ffmpeg APIs.

Supports grabbing a single window or an area of the screen, including
support for multiple monitors (Windows does funky stuff with negative
coordinates here).

I've moved most of the configuration to AVOptions; the input file name
is now only the string "desktop", or "title=<windowname>" to select a
single window. The AVOptions are the same as x11grab where possible.

Code has been added to support a "show_region" mode, like x11grab, which
will draw a rectangle on the screen around the area being captured.

Instead of duplicating code for paletted image handling, I make use of
the GDI API's ability to output DIB (BMP) images, which can be run
through ffmpeg's existing BMP decoder.

Signed-off-by: Calvin Walton <calvin.walton@kepstin.ca>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/general.texi

@@ -1088,6 +1088,7 @@ performance on systems without hardware floating point support).
 @item Video4Linux2      @tab X      @tab X
 @item VfW capture       @tab X      @tab
 @item X11 grabbing      @tab X      @tab
+@item Win32 grabbing    @tab X      @tab
 @end multitable
 
 @code{X} means that input/output is supported.

doc/indevs.texi

@@ -192,6 +192,81 @@ ffmpeg -f fbdev -frames:v 1 -r 1 -i /dev/fb0 screenshot.jpeg
 
 See also @url{http://linux-fbdev.sourceforge.net/}, and fbset(1).
 
+@section gdigrab
+
+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:
+@example
+desktop
+@end example
+or
+@example
+title=@var{window_title}
+@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
+window, regardless of its position on the screen.
+
+For example, to grab the entire desktop using @command{ffmpeg}:
+@example
+ffmpeg -f gdigrab -framerate 6 -i desktop out.mpg
+@end example
+
+Grab a 640x480 region at position @code{10,20}:
+@example
+ffmpeg -f gdigrab -framerate 6 -offset_x 10 -offset_y 20 -video_size vga -i desktop out.mpg
+@end example
+
+Grab the contents of the window named "Calculator"
+@example
+ffmpeg -f gdigrab -framerate 6 -i title=Calculator out.mpg
+@end example
+
+@subsection Options
+
+@table @option
+@item draw_mouse
+Specify whether to draw the mouse pointer. Use the value @code{0} to
+not draw the pointer. Default value is @code{1}.
+
+@item framerate
+Set the grabbing frame rate. Default value is @code{ntsc},
+corresponding to a frame rate of @code{30000/1001}.
+
+@item show_region
+Show grabbed region on screen.
+
+If @var{show_region} is specified with @code{1}, then the grabbing
+region will be indicated on screen. With this option, it is easy to
+know what is being grabbed if only a portion of the screen is grabbed.
+
+Note that @var{show_region} is incompatible with grabbing the contents
+of a single window.
+
+For example:
+@example
+ffmpeg -f gdigrab -show_region 1 -framerate 6 -video_size cif -offset_x 10 -offset_y 20 -i desktop out.mpg
+@end example
+
+@item video_size
+Set the video frame size. The default is to capture the full screen if @file{desktop} is selected, or the full window size if @file{title=@var{window_title}} is selected.
+
+@item offset_x
+When capturing a region with @var{video_size}, set the distance from the left edge of the screen or desktop.
+
+Note that the offset calculation is from the top left corner of the primary monitor on Windows. If you have a monitor positioned to the left of your primary monitor, you will need to use a negative @var{offset_x} value to move the region to that monitor.
+
+@item offset_y
+When capturing a region with @var{video_size}, set the distance from the top edge of the screen or desktop.
+
+Note that the offset calculation is from the top left corner of the primary monitor on Windows. If you have a monitor positioned above your primary monitor, you will need to use a negative @var{offset_y} value to move the region to that monitor.
+
+@end table
+
 @section iec61883
 
 FireWire DV/HDV input device using libiec61883.