use WebPSafe[CM]alloc/WebPSafeFree instead of [cm]alloc/free

there's still some malloc/free in the external example
This is an encoder API change because of the introduction
of WebPMemoryWriterClear() for symmetry reasons.

The MemoryWriter object should probably go in examples/ instead
of being in the main lib, though.
mux_types.h stil contain some inlined free()/malloc() that are
harder to remove (we need to put them in the libwebputils lib
and make sure link is ok). Left as a TODO for now.

Also: WebPDecodeRGB*() function are still returning a pointer
that needs to be free()'d. We should call WebPSafeFree() on
these, but it means exposing the whole mechanism. TODO(later).

Change-Id: Iad2c9060f7fa6040e3ba489c8b07f4caadfab77b

examples/cwebp.c

@@ -1121,7 +1121,7 @@ int main(int argc, const char *argv[]) {
   return_value = 0;
 
  Error:
-  free(memory_writer.mem);
+  WebPMemoryWriterClear(&memory_writer);
   free(picture.extra_info);
   MetadataFree(&metadata);
   WebPPictureFree(&picture);