When replaying resource discards fill with explicit pattern. Closes #284

* This helps catches cases where a discarded image is accidentally used and in
  many cases may still have valid data. Particularly on Vulkan this is relevant
  for DONT_CARE renderpass load and store ops.

renderdoc/api/replay/rdcstr.h

@@ -48,6 +48,8 @@ class rdcliteral
 public:
   const char *c_str() const { return str; }
   size_t length() const { return len; }
+  const char *begin() const { return str; }
+  const char *end() const { return str + len; }
 };
 
 inline rdcliteral operator"" _lit(const char *str, size_t len)