LibGfx: Use enum instead of magic numbers for PNG Color and Filter types

Userland/Libraries/LibGfx/PNGShared.h

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2022, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace Gfx::PNG {
+
+// https://www.w3.org/TR/PNG/#6Colour-values
+enum class ColorType : u8 {
+    Greyscale = 0,
+    Truecolor = 2, // RGB
+    IndexedColor = 3,
+    GreyscaleWithAlpha = 4,
+    TruecolorWithAlpha = 6,
+};
+
+// https://www.w3.org/TR/PNG/#9Filter-types
+enum class FilterType : u8 {
+    None,
+    Sub,
+    Up,
+    Average,
+    Paeth,
+};
+
+};