libobs/util: Add function to get free space

Meant as a part of solving mantis issue 105 ("Disk space usage monitor
when recording").

From pull request: jp9000/obs-studio#374
Relevant mantis issue: https://obsproject.com/mantis/view.php?id=105

libobs/util/platform-windows.c

@@ -362,6 +362,25 @@ void os_closedir(os_dir_t *dir)
 	}
 }
 
+int64_t os_get_free_space(const char *path)
+{
+	ULARGE_INTEGER  remainingSpace;
+	char            abs_path[512];
+	wchar_t         w_abs_path[512];
+
+	if (os_get_abs_path(path, abs_path, 512) > 0) {
+		if (os_utf8_to_wcs(abs_path, 0, w_abs_path, 512) > 0) {
+			BOOL success = GetDiskFreeSpaceExW(w_abs_path,
+					(PULARGE_INTEGER)&remainingSpace,
+					NULL, NULL);
+			if (success)
+				return (int64_t)remainingSpace.QuadPart;
+		}
+	}
+
+	return -1;
+}
+
 static void make_globent(struct os_globent *ent, WIN32_FIND_DATA *wfd,
 		const char *pattern)
 {