LibGfx/TIFF: Skip channels that we are unable to interpret

As per the specification, TIFF readers should gracefully skip samples
that they are not able to interpret.

This patch allow us to read `strike.tif` from the libtiff test suite as
an RGB image.

Userland/Libraries/LibGfx/ImageFormats/TIFFLoader.cpp

@@ -87,13 +87,38 @@ private:
         return NumericLimits<u8>::max() * value / ((1 << bits) - 1);
     }
 
+    u8 samples_for_photometric_interpretation() const
+    {
+        switch (*m_metadata.photometric_interpretation()) {
+        case PhotometricInterpretation::WhiteIsZero:
+        case PhotometricInterpretation::BlackIsZero:
+            return 1;
+        case PhotometricInterpretation::RGB:
+            return 3;
+        default:
+            TODO();
+        }
+    }
+
     ErrorOr<Color> read_color(BigEndianInputBitStream& stream)
     {
         auto bits_per_sample = *m_metadata.bits_per_sample();
+
+        // Section 7: Additional Baseline TIFF Requirements
+        // Some TIFF files may have more components per pixel than you think. A Baseline TIFF reader must skip over
+        // them gracefully, using the values of the SamplesPerPixel and BitsPerSample fields.
+        auto discard_unknown_channels = [&]() -> ErrorOr<void> {
+            auto const unknown_channels = *m_metadata.samples_per_pixel() - samples_for_photometric_interpretation();
+            for (u8 i = bits_per_sample.size() - unknown_channels; i < bits_per_sample.size(); ++i)
+                TRY(read_component(stream, bits_per_sample[i]));
+            return {};
+        };
+
         if (m_metadata.photometric_interpretation() == PhotometricInterpretation::RGB) {
             auto const first_component = TRY(read_component(stream, bits_per_sample[0]));
             auto const second_component = TRY(read_component(stream, bits_per_sample[1]));
             auto const third_component = TRY(read_component(stream, bits_per_sample[2]));
+            TRY(discard_unknown_channels());
             return Color(first_component, second_component, third_component);
         }
 
@@ -104,6 +129,7 @@ private:
             if (m_metadata.photometric_interpretation() == PhotometricInterpretation::WhiteIsZero)
                 luminosity = ~luminosity;
 
+            TRY(discard_unknown_channels());
             return Color(luminosity, luminosity, luminosity);
         }