Fix MT04059: Add new method reset_origin() to screen devices to allow

manual synchronization of VBLANK start against an external timing source.
Updated the MC6845 device to call reset_origin() on its screen at the
start of each frame if a screen is present.

The practical upshot is that now the screen timing and MC6845 timing is
once against synchronized, but by tying the screen timing to the MC6845
and not the other way around.

src/emu/video.c

@@ -2087,6 +2087,23 @@ void screen_device::configure(int width, int height, const rectangle &visarea, a
 }
 
 
+//-------------------------------------------------
+//  reset_origin - reset the timing such that the
+//  given (x,y) occurs at the current time
+//-------------------------------------------------
+
+void screen_device::reset_origin(int beamy, int beamx)
+{
+	// compute the effective VBLANK start/end times
+	m_vblank_end_time = attotime_sub_attoseconds(timer_get_time(machine), beamy * m_scantime + beamx * m_pixeltime);
+	m_vblank_start_time = attotime_sub_attoseconds(m_vblank_end_time, m_vblank_period);
+
+	// re-adjust the scanline 0 and VBLANK timers
+	timer_adjust_oneshot(m_scanline0_timer, time_until_pos(0), 0);
+	timer_adjust_oneshot(m_vblank_begin_timer, time_until_vblank_start(), 0);
+}
+
+
 //-------------------------------------------------
 //  realloc_screen_bitmaps - reallocate bitmaps
 //  and textures as necessary

src/emu/video.h

@@ -160,6 +160,7 @@ public:
 
 	// dynamic configuration
 	void configure(int width, int height, const rectangle &visarea, attoseconds_t frame_period);
+	void reset_origin(int beamy = 0, int beamx = 0);
 	void set_visible_area(int min_x, int max_x, int min_y, int max_y);
 
 	// beam positioning and state

src/emu/video/mc6845.c

@@ -651,6 +651,9 @@ static TIMER_CALLBACK( line_timer_cb )
 			mc6845->line_enable_ff = 1;
 			/* also update the cursor state now */
 			update_cursor_state(mc6845);
+
+			if (mc6845->screen != NULL)
+				mc6845->screen->reset_origin();
 		}
 		else
 		{