Some Kindle screen refresh experiments...

Related to  [#550](/koreader/koreader/issues/550)
according to:

```
/*
 * Are we using FB or an alternate (overlay)
 * buffer for source of update?
 */
if (upd_data->flags & EPDC_FLAG_USE_ALT_BUFFER) {
	src_width = upd_data->alt_buffer_data.width;
	src_height = upd_data->alt_buffer_data.height;
	src_upd_region = &upd_data->alt_buffer_data.alt_update_region;
} else {
	src_width = fb_data->epdc_fb_var.xres_virtual;
	src_height = fb_data->epdc_fb_var.yres;
	src_upd_region = &upd_data->update_region;
}
```

and

```
if (upd_data->flags & EPDC_FLAG_USE_ALT_BUFFER) {
	sg_dma_address(&fb_data->sg[0]) =
		upd_data->alt_buffer_data.phys_addr + pxp_input_offs;
} else {
	sg_dma_address(&fb_data->sg[0]) =
		fb_data->info.fix.smem_start + fb_data->fb_offset + pxp_input_offs;
	sg_set_page(&fb_data->sg[0],
				virt_to_page(fb_data->info.screen_base),
				fb_data->info.fix.smem_len,
				offset_in_page(fb_data->info.screen_base));
}
```

Yes.