fix multiple allocation for transform buffer

We were not updating the current_width_, which is usually
not a problem, unless we use Delta Palette with small number
of colors
-> Addressed this re-entrancy problem by checking we have
enough capacity for transform buffer.

The problem is not currently visible, until we restrict
the number of gradient used in delta-palette to less than 16.
Then the buffers have different current_width_ and the problem
surfaces.

Change-Id: Icd84b919905d7789014bb6668bfb6813c93fb36e

src/enc/vp8li.h

@@ -32,6 +32,9 @@ typedef struct {
   uint32_t* argb_scratch_;      // Scratch memory for argb rows
                                 // (used for prediction).
   uint32_t* transform_data_;    // Scratch memory for transform data.
+  uint32_t* transform_mem_;       // Currently allocated memory.
+  size_t    transform_mem_size_;  // Currently allocated memory size.
+
   int       current_width_;     // Corresponds to packed image width.
 
   // Encoding parameters derived from quality parameter.