LibGfx: Change return type of Adler32 checksums in PNGWriter

The two Adler32 checksums are u16 and these two getters were mistakenly
left as u32 when PNGChunk::add_as_big_endian() was templated leading
to corrupted IDAT fields in our PNGs.

Userland/Libraries/LibGfx/PNGWriter.cpp

@@ -51,8 +51,8 @@ public:
     void finalize(PNGChunk&);
     void add_byte_to_block(u8 data, PNGChunk&);
 
-    u32 adler_s1() const { return m_adler_s1; }
-    u32 adler_s2() const { return m_adler_s2; }
+    u16 adler_s1() const { return m_adler_s1; }
+    u16 adler_s2() const { return m_adler_s2; }
 
 private:
     void add_block_to_chunk(PNGChunk&, bool);