[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/main.cpp

@@ -60,6 +60,7 @@ main(int argc, const char **argv)
 	screen::pixel_t fg = scr.color(0xb0, 0xb0, 0xb0);
 	screen::pixel_t bg = scr.color(49, 79, 128);
 	scr.fill(bg);
+	scr.update();
 
 	constexpr int margin = 2;
 	const unsigned xstride = (margin + fnt.width());
@@ -85,11 +86,13 @@ main(int argc, const char **argv)
 			scroll.add_line(e->message, eom);
 			if (++pending > pending_threshold) {
 				scroll.render(scr, fnt);
+				scr.update();
 				pending = 0;
 			}
 		} else {
 			if (pending) {
 				scroll.render(scr, fnt);
+				scr.update();
 				pending = 0;
 			}
 		}

src/drivers/fb/screen.cpp

@@ -1,3 +1,5 @@
+#include <stdlib.h>
+#include <string.h>
 #include "screen.h"
 
 screen::screen(void *addr, unsigned hres, unsigned vres, pixel_order order) :
@@ -6,6 +8,7 @@ screen::screen(void *addr, unsigned hres, unsigned vres, pixel_order order) :
 	m_resx(hres),
 	m_resy(vres)
 {
+	m_back = reinterpret_cast<pixel_t*>(malloc(hres*vres*sizeof(pixel_t)));
 }
 
 screen::pixel_t
@@ -31,11 +34,17 @@ screen::fill(pixel_t color)
 {
 	const size_t len = m_resx * m_resy;
 	for (size_t i = 0; i < len; ++i)
-		m_fb[i] = color;
+		m_back[i] = color;
 }
 
 void
 screen::draw_pixel(unsigned x, unsigned y, pixel_t color)
 {
-	m_fb[x + y * m_resx] = color;
+	m_back[x + y * m_resx] = color;
+}
+
+void
+screen::update()
+{
+	memcpy(m_fb, m_back, m_resx*m_resy*sizeof(pixel_t));
 }

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;