LibGfx: Implement PNG filtering on write

Is it another great upgrade to our PNG encoder like in 9aafaec259?
Well, not really - it's not a 2x or 55x improvement like you saw there,
but still it saves something:

- a screenshot of a blank Serenity desktop dropped from about 45 KiB
  to 40 KiB.
- re-encoding NASA photo of the Earth to PNG again saves about 25%
  (16.5 MiB -> 12.3 MiB), compared to not using filters.

[1]: https://commons.wikimedia.org/wiki/File:The_Blue_Marble_(remastered).jpg

AK/SIMDExtras.h

@@ -36,6 +36,18 @@ ALWAYS_INLINE static constexpr u32x4 expand4(u32 u)
 
 // Casting
 
+template<typename TSrc>
+ALWAYS_INLINE static u8x4 to_u8x4(TSrc v)
+{
+    return __builtin_convertvector(v, u8x4);
+}
+
+template<typename TSrc>
+ALWAYS_INLINE static u16x4 to_u16x4(TSrc v)
+{
+    return __builtin_convertvector(v, u16x4);
+}
+
 template<typename TSrc>
 ALWAYS_INLINE static u32x4 to_u32x4(TSrc v)
 {