[fb] Use double-buffering in fb driver

Allocate and use a back buffer, so that draws to the screen are always a
single memcpy()

src/drivers/fb/screen.h

@@ -19,8 +19,10 @@ public:
 	void fill(pixel_t color);
 	void draw_pixel(unsigned x, unsigned y, pixel_t color);
 
+	void update();
+
 private:
-	pixel_t *m_fb;
+	pixel_t *m_fb, *m_back;
 	pixel_order m_order;
 	unsigned m_resx, m_resy;