vp8: align left pixel array by 16 bytes.

The x86 simd expects this. Identical alignment can be found in vp9
and vp10 also. Fixes crashes on 32bit x86 systems.

Change-Id: I229c88d8f696acbef5337c8fa9503528df4e1c40

vp8/common/reconintra.c

@@ -55,7 +55,7 @@ void vp8_build_intra_predictors_mby_s(MACROBLOCKD *x,
                                       int y_stride)
 {
     MB_PREDICTION_MODE mode = x->mode_info_context->mbmi.mode;
-    unsigned char yleft_col[16];
+    DECLARE_ALIGNED(16, uint8_t, yleft_col[16]);
     int i;
     intra_pred_fn fn;