fix skpdiff viewer bug when using relative paths

tools/skpdiff/skpdiff_util.cpp

Index: tools/skpdiff/skpdiff_util.cpp
diff --git a/tools/skpdiff/skpdiff_util.cpp b/tools/skpdiff/skpdiff_util.cpp
index 0047959b60fbda4496c868c613240839d0f9b6b8..e06a314d5c6d56de62588f6506443dcc22736022 100644
--- a/tools/skpdiff/skpdiff_util.cpp
+++ b/tools/skpdiff/skpdiff_util.cpp
@@ -15,10 +15,15 @@
 #   include <glob.h>
 #endif
 
+#if SK_BUILD_FOR_MAC
+#   include <sys/syslimits.h> // PATH_MAX is here for Macs
+#endif
+
 #if SK_BUILD_FOR_WIN32
 #   include <windows.h>
 #endif
 
+#include <stdlib.h>
 #include <time.h>
 #include "SkOSFile.h"
 #include "skpdiff_util.h"
@@ -181,3 +186,15 @@ bool glob_files(const char globPattern[], SkTArray<SkString>* entries) {
     return false;
 #endif
 }
+
+char* get_absolute_path(const char path[]) {
+#if SK_BUILD_FOR_MAC || SK_BUILD_FOR_UNIX || SK_BUILD_FOR_ANDROID
+    char* fullPath = (char*)malloc(PATH_MAX+1);
+    return realpath(path, fullPath); // On failure this returns NULL, and fullPath on success.
+#elif SK_BUILD_FOR_WIN32
+    // By passing in NULL, we are asking windows to malloc the memory for us. The 0 is ignored.
+    return _fullpath(NULL, path, 0);
+#else
+    return NULL
+#endif
+}