LibGfx: Let decode_bmp_v5_dib() skip reserved field

This has no effect in practice: decode_bmp_v5_dib() is the last
thing called with the streamer object (...the streamer is passed
to set_dib_bitmasks(), but that doesn't read anything off it except
for DIBType::Info bitmaps, which v5 bitmaps aren't).

But dib_size is 16 larger for v5 than for v4, so we should read
16 bytes.

This is also useful for a hypothetical person who might look at
the reading code to figure out how the writing code should look like.

Userland/Libraries/LibGfx/BMPLoader.cpp

@@ -805,6 +805,7 @@ static bool decode_bmp_v5_dib(BMPLoadingContext& context, InputStreamer& streame
     v5.intent = streamer.read_u32();
     v5.profile_data = streamer.read_u32();
     v5.profile_size = streamer.read_u32();
+    streamer.drop_bytes(4); // Ignore reserved field.
 
     if constexpr (BMP_DEBUG) {
         dbgln("BMP intent: {}", v5.intent);