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/win32_platform.h

@@ -277,8 +277,7 @@ typedef struct _GLFWcursorWin32
 typedef struct _GLFWtimeWin32
 {
     GLFWbool            hasPC;
-    double              resolution;
-    unsigned __int64    base;
+    GLFWuint64          frequency;
 
 } _GLFWtimeWin32;