ubb-vga.c (delay): try to compensate for APB jitter (experimental)

First, we extend all deadlines a litte to make sure we never have to
wait negative time. Second, we get the timer value at the end of the
wait and CPU-loop for the difference between the actual value and the
ideal (extended) value.

ubb-vga/ubb-vga.c

@@ -244,11 +244,17 @@ static void cleanup(void)
 static void delay(uint16_t cycles)
 {
 	static uint16_t next = 0;
+	uint16_t t;
 
 	next += cycles;
 	TDHR(TIMER) = next;
 	TFCR = 1 << (TIMER+16);
 	while (!(TFR & (1 << (TIMER+16))));
+	t = next+24-TCNT(TIMER);
+	t >>= 1;
+	while (t--)
+		asm("");
+	
 }