Merge pull request #25661 from itlab-vision:framebuffer

Highgui backend on top of Framebuffer #25661

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [ x] I agree to contribute to the project under Apache 2 License.
- [ x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [ x] The PR is proposed to the proper branch
- [ ] There is a reference to the original bug report and related work
- [ x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ x] The feature is well documented and sample code can be built with the project CMake

Environment variables used:
OPENCV_UI_BACKEND - you need to add the value “FB”
OPENCV_UI_PRIORITY_FB - requires priority indication
OPENCV_HIGHGUI_FB_MODE={FB|XVFB|EMU} - mode of using Framebuffer (default "FB")
- FB - Linux Framebuffer
- XVFB - virtual Framebuffer
- EMU - emulation (images are not displayed)
OPENCV_HIGHGUI_FB_DEVICE (FRAMEBUFFER) - path to the Framebuffer file (default "/dev/fb0").

Examples of using:

sudo OPENCV_UI_BACKEND=FB ./opencv_test_highgui
sudo OPENCV_UI_PRIORITY_FB=1111 ./opencv_test_highgui
OPENCV_UI_BACKEND=FB OPENCV_HIGHGUI_FB_MODE=EMU ./opencv_test_highgui
sudo OPENCV_UI_BACKEND=FB OPENCV_HIGHGUI_FB_MODE=FB ./opencv_test_highgui

export DISPLAY=:99
Xvfb $DISPLAY -screen 0 1024x768x24 -fbdir /tmp/ -f /tmp/user.xvfb.auth&
sudo -u sipeed XAUTHORITY=/tmp/user.xvfb.auth x11vnc -display $DISPLAY -listen localhost&
DISPLAY=:0 gvncviewer localhost&

FRAMEBUFFER=/tmp/Xvfb_screen0 OPENCV_UI_BACKEND=FB OPENCV_HIGHGUI_FB_MODE=XVFB ./opencv_test_highgui

cmake/checks/framebuffer.cpp

@@ -0,0 +1,9 @@
+#include <X11/XWDFile.h>
+#include <X11/X.h>
+
+int main(void)
+{
+    XWDFileHeader *xwd_header;
+    XWDColor *xwd_colors;
+    return 0;
+}