[client] x11: change to a more visible "dot" cursor

This change alters the small square dot cursor to a more visible 16x16
cursor for X11. A new option `spice:largeDotCursor` can be set to use an
alternative 32x32 cursor for the vision impaired.

client/displayservers/X11/cursor.c

@@ -0,0 +1,105 @@
+/**
+ * Looking Glass
+ * Copyright © 2017-2023 The Looking Glass Authors
+ * https://looking-glass.io
+ *
+ * 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., 59
+ * Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+#include "cursor.h"
+#include "common/util.h"
+
+#include <string.h>
+#include <errno.h>
+
+struct MemFile
+{
+  const char * data;
+  int          size;
+  int          pos;
+};
+
+static int x11cursor_read(XcursorFile *file, unsigned char * buf, int len)
+{
+  struct MemFile * f = (struct MemFile *)file->closure;
+
+  if (f->pos == f->size)
+    return 0;
+
+  len = min(f->size - f->pos, len);
+  memcpy(buf, f->data + f->pos, len);
+  f->pos += len;
+
+  return len;
+}
+
+static int x11cursor_write(XcursorFile *file, unsigned char * buf, int len)
+{
+  errno = -EINVAL;
+  return -1;
+}
+
+static int x11cursor_seek(XcursorFile *file, long offset, int whence)
+{
+  struct MemFile * f = (struct MemFile *)file->closure;
+  long target;
+
+  switch(whence)
+  {
+    case SEEK_SET:
+      target = offset;
+      break;
+
+    case SEEK_CUR:
+      target = f->pos + offset;
+      break;
+
+    case SEEK_END:
+      target = f->size + offset;
+      break;
+
+    default:
+      errno = -EINVAL;
+      return -1;
+  }
+
+  if (target < 0 || target > f->size)
+  {
+    errno = -EINVAL;
+    return -1;
+  }
+  f->pos = target;
+  return target;
+}
+
+XcursorImages * x11cursor_load(const char * cursor, int size)
+{
+  struct MemFile closure =
+  {
+    .data = cursor,
+    .size = size,
+    .pos  = 0
+  };
+
+  XcursorFile f =
+  {
+    .closure = &closure,
+    .read    = x11cursor_read,
+    .write   = x11cursor_write,
+    .seek    = x11cursor_seek
+  };
+
+  return XcursorXcFileLoadAllImages(&f);
+}