Codechange: introduce SpriteFile to be used by the sprite loader instead of the global FIO slot functionality

src/spriteloader/sprite_file.cpp

@@ -0,0 +1,50 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file sprite_file.cpp Implementation of logic specific to the SpriteFile class. */
+
+#include "../stdafx.h"
+#include "sprite_file_type.hpp"
+
+/** Signature of a container version 2 GRF. */
+extern const byte _grf_cont_v2_sig[8] = {'G', 'R', 'F', 0x82, 0x0D, 0x0A, 0x1A, 0x0A};
+
+/**
+ * Get the container version of the currently opened GRF file.
+ * @return Container version of the GRF file or 0 if the file is corrupt/no GRF file.
+ */
+static byte GetGRFContainerVersion(SpriteFile &file)
+{
+	size_t pos = file.GetPos();
+
+	if (file.ReadWord() == 0) {
+		/* Check for GRF container version 2, which is identified by the bytes
+		 * '47 52 46 82 0D 0A 1A 0A' at the start of the file. */
+		for (uint i = 0; i < lengthof(_grf_cont_v2_sig); i++) {
+			if (file.ReadByte() != _grf_cont_v2_sig[i]) return 0; // Invalid format
+		}
+
+		return 2;
+	}
+
+	/* Container version 1 has no header, rewind to start. */
+	file.SeekTo(pos, SEEK_SET);
+	return 1;
+}
+
+/**
+ * Create the SpriteFile.
+ * @param filename      Name of the file at the disk.
+ * @param subdir        The sub directory to search this file in.
+ * @param palette_remap Whether a palette remap needs to be performed for this file.
+ */
+SpriteFile::SpriteFile(const std::string &filename, Subdirectory subdir, bool palette_remap)
+	: RandomAccessFile(filename, subdir), palette_remap(palette_remap)
+{
+	this->container_version = GetGRFContainerVersion(*this);
+	this->content_begin = this->GetPos();
+}