MacCreadyVopt: ignore failed GlideResults

Fixes valgrind warnings and potential calculation bugs.
OpenGL/Extension: remove space check

This check is neither necessary nor helpful.
OpenGL/Extension: return true or false
OpenGL/Extension: make variables more local
OpenGL/Extension: add nullptr check for Android

Workaround for broken graphics drivers.
GlideSolvers/MacCreadyVopt: check for positive distance

Workaround for potential division by zero.
Revert "AirspaceRenderer: Enable the explicit use of airspace paddings on PPC200x"

This reverts commit 767460365c6f1d486d4699405e5403263b618adf.

The commit has never worked.  It causes an assertion failure in
Canvas::ClearWhite() whenever a user with a PPC2000 build attempts to
enable airspace paddings, because the stencil buffer is not available.
release v6.4.6