sandbox: Add --video_frames option to capture test frames

Add a new --video_frames option to sandbox which accepts a directory path.
When set, every video test assertion writes a BMP file (frame0.bmp,
frame1.bmp, etc.) to the specified directory, allowing visual inspection
of the framebuffer at each test step.

A new video_write_bmp() function writes 16/32bpp framebuffer contents as
Windows BMP files. On startup, any existing frame*.bmp files in the
directory are removed to ensure a clean state.

Usage example:

  ./u-boot --video_frames /tmp/frames -c "ut dm video_text"

Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <sjg@chromium.org>

doc/arch/sandbox/sandbox.rst

@@ -252,6 +252,9 @@ available options. Some of these are described below:
 -v, --verbose
   Show console output from tests. Normally this is suppressed.
 
+--video_frames <dir>
+  Write video frames to the specified directory for debugging purposes.
+
 -V, --video_test <ms>
   Enable video test mode with a delay (in milliseconds) between assertions. This
   allows time to examine the display during testing.