dsp/lossless: split enc/dec functions

adds lossless_enc*.c; reduces the size of the decode-only so: ~78K
w/gcc-4.8.2 on x86_64.

Change-Id: If5e4610b67d05eba5896bc64bab79e9df92b2092

src/dsp/lossless.h

@@ -89,6 +89,7 @@ void VP8LAddGreenToBlueAndRed_C(uint32_t* data, int num_pixels);
 
 // Must be called before calling any of the above methods.
 void VP8LDspInit(void);
+void VP8LEncDspInit(void);
 
 //------------------------------------------------------------------------------
 // Image transforms.