Fixes to gears.pp example

[michaliskambi]

1. error_callback needs a fix to compile, otherwise I get this:

$ fpc gears.pp -Fu../
Free Pascal Compiler version 3.2.2-rrelease_3_2_2-0-g0d122c4953 [2022/05/16] for x86_64
Copyright (c) 1993-2021 by Florian Klaempfl and others
Target OS: Darwin for x86_64
Compiling gears.pp
....
gears.pp(291,40) Error: Incompatible type for arg no. 1: Got "<address of procedure(const LongInt;const ShortString);Register>", expected "<procedure variable type of procedure(LongInt;ShortString);Register>"
gears.pp(316) Fatal: There were 1 errors compiling module, stopping
Fatal: Compilation aborted

2. In this fork, I see one has to use CThreads on Unix.

3. And a tiny typo fix - weird word wglpt in the comment, should be just want looking at gears original C code ( https://github.com/freedesktop/mesa-demos/blob/master/src/demos/gears.c ) :)