LibGfx: Add a .pam loader

.pam is a "portrable arbitrarymap" as documented at
https://netpbm.sourceforge.net/doc/pam.html

It's very similar to .pbm, .pgm, and .ppm, so this uses the
PortableImageMapLoader framework. The header is slightly different,
so this has a custom header parsing function.

Also, .pam only exixts in binary form, so the ascii form support
becomes optional.

Userland/Libraries/LibGfx/ImageFormats/PAMLoader.h

@@ -0,0 +1,92 @@
+/*
+ * Copyright (c) 2024, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/StringView.h>
+#include <LibGfx/ImageFormats/ImageDecoder.h>
+#include <LibGfx/ImageFormats/PortableImageMapLoader.h>
+
+namespace Gfx {
+
+struct PAM {
+    static constexpr auto binary_magic_number = '7';
+    static constexpr StringView image_type = "PAM"sv;
+    u16 max_val { 0 };
+    u16 depth { 0 };
+    String tupl_type {};
+};
+
+using PAMLoadingContext = PortableImageMapLoadingContext<PAM>;
+
+template<class Context>
+ErrorOr<void> read_pam_header(Context& context)
+{
+    // https://netpbm.sourceforge.net/doc/pam.html
+    TRY(read_magic_number(context));
+
+    Optional<u16> width;
+    Optional<u16> height;
+    Optional<u16> depth;
+    Optional<u16> max_val;
+    Optional<String> tupltype;
+
+    while (true) {
+        TRY(read_whitespace(context));
+
+        auto const token = TRY(read_token(*context.stream));
+
+        if (token == "ENDHDR") {
+            auto newline = TRY(context.stream->template read_value<u8>());
+            if (newline != '\n')
+                return Error::from_string_view("PAM ENDHDR not followed by newline"sv);
+            break;
+        }
+
+        TRY(read_whitespace(context));
+        if (token == "WIDTH") {
+            if (width.has_value())
+                return Error::from_string_view("Duplicate PAM WIDTH field"sv);
+            width = TRY(read_number(*context.stream));
+        } else if (token == "HEIGHT") {
+            if (height.has_value())
+                return Error::from_string_view("Duplicate PAM HEIGHT field"sv);
+            height = TRY(read_number(*context.stream));
+        } else if (token == "DEPTH") {
+            if (depth.has_value())
+                return Error::from_string_view("Duplicate PAM DEPTH field"sv);
+            depth = TRY(read_number(*context.stream));
+        } else if (token == "MAXVAL") {
+            if (max_val.has_value())
+                return Error::from_string_view("Duplicate PAM MAXVAL field"sv);
+            max_val = TRY(read_number(*context.stream));
+        } else if (token == "TUPLTYPE") {
+            // FIXME: tupltype should be all text until the next newline, with leading and trailing space stripped.
+            // FIXME: If there are multipe TUPLTYPE lines, their values are all appended.
+            tupltype = TRY(read_token(*context.stream));
+        } else {
+            return Error::from_string_view("Unknown PAM token"sv);
+        }
+    }
+
+    if (!width.has_value() || !height.has_value() || !depth.has_value() || !max_val.has_value())
+        return Error::from_string_view("Missing PAM header fields"sv);
+    context.width = *width;
+    context.height = *height;
+    context.format_details.depth = *depth;
+    context.format_details.max_val = *max_val;
+    if (tupltype.has_value())
+        context.format_details.tupl_type = *tupltype;
+
+    context.state = Context::State::HeaderDecoded;
+
+    return {};
+}
+
+using PAMImageDecoderPlugin = PortableImageDecoderPlugin<PAMLoadingContext>;
+
+ErrorOr<void> read_image_data(PAMLoadingContext& context);
+}