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.

glfw/cocoa_platform.h

@@ -144,7 +144,7 @@ typedef struct _GLFWDisplayLinkNS
 {
     CVDisplayLinkRef displayLink;
     CGDirectDisplayID displayID;
-    double lastRenderFrameRequestedAt;
+    monotonic_t lastRenderFrameRequestedAt;
 } _GLFWDisplayLinkNS;
 
 // Cocoa-specific global data