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

src/enc/histogram.h

@@ -46,7 +46,7 @@ typedef struct {
 } VP8LHistogram;
 
 // Collection of histograms with fixed capacity, allocated as one
-// big memory chunk. Can be destroyed by simply calling 'free()'.
+// big memory chunk. Can be destroyed by calling WebPSafeFree().
 typedef struct {
   int size;         // number of slots currently in use
   int max_size;     // maximum capacity