Add glfwGetTimerValue and glfwGetTimerFrequency

This adds raw timer access to the public API and builds the
floating-point time functions on top.  It also makes the GLFWuint64 type
public.

src/cocoa_platform.h

@@ -118,8 +118,7 @@ typedef struct _GLFWcursorNS
 //
 typedef struct _GLFWtimeNS
 {
-    double          base;
-    double          resolution;
+    GLFWuint64      frequency;
 
 } _GLFWtimeNS;