makepanda: Record cache timestamps as integers rather than floats

We don't need the extra precision, in fact it is detrimental to restoring build caches in a cross-platform way.

This commit will invalidate all current build caches.

Cherry-picked from 2a904f398592ce7effedc4f12720be0cef9b6cc9 (see #1260)

makepanda/makepandacore.py

@@ -714,8 +714,10 @@ TIMESTAMPCACHE = {}
 def GetTimestamp(path):
     if path in TIMESTAMPCACHE:
         return TIMESTAMPCACHE[path]
-    try: date = os.path.getmtime(path)
-    except: date = 0
+    try:
+        date = int(os.path.getmtime(path))
+    except:
+        date = 0
     TIMESTAMPCACHE[path] = date
     return date
 
@@ -866,7 +868,7 @@ def JavaGetImports(path):
 ##
 ########################################################################
 
-DCACHE_VERSION = 2
+DCACHE_VERSION = 3
 DCACHE_BACKED_UP = False
 
 def SaveDependencyCache():