Implement frame buffer double-buffering

This slowed down drawing to video in QEMU/VirtualBox but greatly sped up
doing so on real hardware (no longer reading from video memory).
Despite the speed-up, it is still quite slow though.

src/hulk/hulk.d

@@ -57,7 +57,11 @@ void hulk_start()
     initialize_cpu();
     gdt.initialize();
     idt.initialize();
-    fb.initialize(cast(uint *)HULK_VIRTUAL_FRAMEBUFFER_ADDRESS, hulk_header.bootinfo.fb.width, hulk_header.bootinfo.fb.height, hulk_header.bootinfo.fb.stride);
+    fb.initialize(cast(uint *)HULK_VIRTUAL_FRAMEBUFFER_ADDRESS,
+            cast(uint *)hulk_header.bootinfo.fb_buffer1_phys,
+            hulk_header.bootinfo.fb.width,
+            hulk_header.bootinfo.fb.height,
+            hulk_header.bootinfo.fb.stride);
     console.initialize();
     console.clear();
     klog.initialize();