set GLEW_MX when multicontext is enabled
multicontext support for [gemglxwindow]

LATER: check for memleaks when create/destroy'ing a window
LATER: check whether this can be done more elgant
dynamic light-allocation

using gem::ContextData()
this requires that requestLight() is called with a valid context,
at least when being called for the first time...
better error output

print erroneous numbers, so we can see what is really going on
request lights at startRendering() time

rather than at creation time...
updated openGL m4-checks