Add gltext.py and update README.md

gltext utilizes wxpython to enable the user to easily display system fonts in an pyopengl environment.
Previous behavior caused dc.GetMultiLineTextExtent to fail on Mac.

Previous behavior did not work on Mac because dc.GetMultiLineTextExtent
does not work there until an object is selected. The wx documentation states:
"...If a bitmap is not given to this constructor then don’t forget to select
a bitmap into the DC before drawing on it." so the Mac behavior is proper
even if this step can be skipped elsewhere without a problem. The fix is
simply to select an empty bitmap first.

Note: This commit was created by Stefan Hacker from a patch sent by the
author and is slightly modified. The author was set for proper attribution.