[MP] Removed GL library dependency.

The OpenGL library is resolved at run time, along with all the function
pointers.
[SP] Removed the GL library dependency

The OpenGL library is resolved at runtime along with all the function
pointers.
[MP] Fix #472

This commit solves the bug in two parts. Firstly, the UI code for
displaying GL extensions was previously not doing bounds checks on an
array which could hold up to only 128 extension names. This has been
written to remove the need for the array and so can handle any number
of extensions.

Secondly, the engine has been modified to truncate the GL extensions
string so that only the 128 extensions are listed. This prevents this bug
from appearing for mods which do not use the updated UI code. The original
extensions string is still kept so that the existence of all extensions
can be checked.