Source: http://jon-martin.com/?p=114

private Texture2D ScaleTexture(Texture2D source,int targetWidth,int targetHeight) {
       Texture2D result=new Texture2D(targetWidth,targetHeight,source.format,true);
       Color[] rpixels=result.GetPixels(0);
       float incX=((float)1/source.width)*((float)source.width/targetWidth);
       float incY=((float)1/source.height)*((float)source.height/targetHeight);
       for(int px=0; px<rpixels.Length; px++) {
               rpixels[px] = source.GetPixelBilinear(incX*((float)px%targetWidth),
                                 incY*((float)Mathf.Floor(px/targetWidth)));
       }
       result.SetPixels(rpixels,0);
       result.Apply();
       return result;
}