Removes the Extension::glxAvailable() from kwinglobals and
implements the functionality in CompositingPrefs, where it
is only needed. There used to be one additional check in
scene_opengl_glx.cpp which is moved into composite.cpp
before the OpenGL Scene is created.
REVIEW: 102002
by checking the graphicssystem at startup
also avoid pixmap and memory leaking on the xrender backend, validate some pointers
on deletion in SceneOpnGL and avoid attempts to render ::isNull pixmaps
Second part of cleaning up the lib directory: the effects library
now lives in libkwineffects/ directory.
For existing effects nothing changes as the install path is unchanged.
The change obsoletes the lib/ directory.
As glplatform.h has not yet been exported I dared to export it and
adjust the places where it is used.
CCMAIL: kwin@kde.org
