ok? in gl-context<%>
Returns #t if this context is available OpenGL drawing,
#f otherwise.
A context is unavailable if OpenGL support is disabled at compile time
or run time, if the context is associated with a
bitmap-dc% with no selected bitmap or with a monochrome
selected bitmap, if the context is for a canvas that no longer
exists, or if there was a low-level error when preparing the context.