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/posix_time.h

@@ -38,8 +38,7 @@
 typedef struct _GLFWtimePOSIX
 {
     GLFWbool    monotonic;
-    double      resolution;
-    uint64_t    base;
+    GLFWuint64  frequency;
 
 } _GLFWtimePOSIX;