LibGfx: Commonize P[BGP]M file loading contexts

Much of the code in PBM, PGM, and PPM image loaders is common. The
contexts are nearly identical. Instead of writing multiple contexts,
write 1 with a template argument to pass in the details of the given
format.

Userland/Libraries/LibGfx/PortableImageMapLoader.h

@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2020, Hüseyin ASLITÜRK <asliturk@hotmail.com>
+ * Copyright (c) 2022, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/RefPtr.h>
+#include <AK/StringView.h>
+#include <AK/Types.h>
+#include <LibGfx/Bitmap.h>
+
+namespace Gfx {
+
+template<class TFormatDetails>
+struct PortableImageMapLoadingContext {
+    using FormatDetails = TFormatDetails;
+
+    enum class Type {
+        Unknown,
+        ASCII,
+        RAWBITS
+    };
+
+    enum class State {
+        NotDecoded = 0,
+        Error,
+        MagicNumber,
+        Width,
+        Height,
+        Maxval,
+        Bitmap,
+        Decoded
+    };
+
+    Type type { Type::Unknown };
+    State state { State::NotDecoded };
+    u8 const* data { nullptr };
+    size_t data_size { 0 };
+    size_t width { 0 };
+    size_t height { 0 };
+    FormatDetails format_details {};
+    RefPtr<Gfx::Bitmap> bitmap;
+};
+
+}