LibGfx/TIFF: Correctly upscale samples with a resolution lower than 8

As pointed out by @nico, while doing a right-shift to downscale is fine,
a left-shift to upscale gives wrong results. As an example, imagine a 2-
bits value containing 3, left-shifting it would give 192 instead of 255.

Userland/Libraries/LibGfx/ImageFormats/TIFFLoader.cpp

@@ -83,7 +83,7 @@ private:
 
         if (bits > 8)
             return value >> (bits - 8);
-        return value << (8 - bits);
+        return NumericLimits<u8>::max() * value / ((1 << bits) - 1);
     }
 
     ErrorOr<Color> read_color(BigEndianInputBitStream& stream)