Round alpha correctly in bilinear image scaling code too

Add 0.5 to round the alpha values correctly too, just as it was already
done in f4c9767b49 for the RGB values.

Closes https://github.com/wxWidgets/wxWidgets/pull/733

src/common/image.cpp

@@ -844,7 +844,7 @@ wxImage wxImage::ResampleBilinear(int width, int height) const
             dst_data += 3;
 
             if ( src_alpha )
-                *dst_alpha++ = static_cast<unsigned char>(a1 * dy1 + a2 * dy);
+                *dst_alpha++ = static_cast<unsigned char>(a1 * dy1 + a2 * dy +.5);
         }
     }