removed deprecated glPushAttrib and glPopAttrib

Since in OpenGl 3 these are deprected, it messed things up while
rendering to an FBO, beacuse scissor test was still enabled. Now we
disable it at the end, let PopStyle take care of the blending and depth
test is disabled by default anyway.
Merge pull request #108 from patrickFuerst/master

removed deprecated glPushAttrib and glPopAttrib