LibGfx: Add a QOI image format encoder

Previously only a QOI image decoder was available on the system. This
commit adds an encoder as well.

For now it only handles images with 4 channels (RGBA).

Userland/Libraries/LibGfx/CMakeLists.txt

@@ -37,6 +37,7 @@ set(SOURCES
     PPMLoader.cpp
     Point.cpp
     QOILoader.cpp
+    QOIWriter.cpp
     Rect.cpp
     ShareableBitmap.cpp
     Size.cpp