libcamera: Give MappedFrameBuffer its own implementation

The MappedFrameBuffer is a convenience feature which sits on top of the
FrameBuffer and facilitates mapping it to CPU accessible memory with
mmap.

This implementation is internal and currently sits in the same internal
files as the internal FrameBuffer, thus exposing those internals to
users of the MappedFramebuffer implementation.

Move the MappedFrameBuffer and MappedBuffer implementation to its own
implementation files, and fix the sources throughout to use that
accordingly.

Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

test/mapped-buffer.cpp

@@ -9,7 +9,7 @@
 
 #include <libcamera/framebuffer_allocator.h>
 
-#include "libcamera/internal/framebuffer.h"
+#include "libcamera/internal/mapped_framebuffer.h"
 
 #include "camera_test.h"
 #include "test.h"