POSIX: Use fstat instead of fseek / ftell to retrieve file stream sizes

fstat is generally faster as is does not cause the IO buffer to be
invalidated / refilled.

Benchmark results for the startup time of the SCI engine with Gabriel
Knight 1 CD:
- Linux, glibc, spinning HDD, fseek/ftell: 140 ms
- Linux, glibc, spinning HDD, fstat: 100 ms
- 3DS, newlib, SD card, fseek/ftell: 68 s
- 3DS, newlib, SD card, fstat: 11 s

1 files changed, 56 insertions, 0 deletions

diff --git a/backends/fs/posix/posix-iostream.cpp b/backends/fs/posix/posix-iostream.cpp
new file mode 100644
index 0000000000..59f14142b5
--- /dev/null
+++ b/backends/fs/posix/posix-iostream.cpp
@@ -0,0 +1,56 @@
+/* ScummVM - Graphic Adventure Engine
+ *
+ * ScummVM is the legal property of its developers, whose names
+ * are too numerous to list here. Please refer to the COPYRIGHT
+ * file distributed with this source distribution.
+ *
+ * This program 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; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program 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 this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ */
+
+#define FORBIDDEN_SYMBOL_ALLOW_ALL
+
+#include "backends/fs/posix/posix-iostream.h"
+
+#include <sys/stat.h>
+
+PosixIoStream *PosixIoStream::makeFromPath(const Common::String &path, bool writeMode) {
+	FILE *handle = fopen(path.c_str(), writeMode ? "wb" : "rb");
+
+	if (handle)
+		return new PosixIoStream(handle);
+
+	return nullptr;
+}
+
+PosixIoStream::PosixIoStream(void *handle) :
+		StdioStream(handle) {
+}
+
+int32 PosixIoStream::size() const {
+	int fd = fileno((FILE *)_handle);
+	if (fd == -1) {
+		return StdioStream::size();
+	}
+
+	// Using fstat to obtain the file size is generally faster than fseek / ftell
+	// because it does not affect the IO buffer.
+	struct stat st;
+	if (fstat(fd, &st) == -1) {
+		return StdioStream::size();
+	}
+
+	return st.st_size;
+}