Use datatype monotonic_t instead of double to keep track of time

The time is stored in a signed 64 bit integer with nanosecond accuracy. This eliminates the possibility of floating-point inaccuracies.
`monotonic_t` can currently hold values large enough to work correctly for more than 200 years into the future.
Using a typedef instead of directly using `int64_t` everywhere will also allow easily changing the datatype in the future should the need arise for more precise or bigger time values.

kitty/graphics.h

@@ -6,6 +6,7 @@
 
 #pragma once
 #include "data-types.h"
+#include "monotonic.h"
 
 typedef struct {
     unsigned char action, transmission_type, compressed, delete_action;
@@ -50,7 +51,7 @@ typedef struct {
 
     ImageRef *refs;
     size_t refcnt, refcap;
-    double atime;
+    monotonic_t atime;
     size_t used_storage;
 } Image;