The opengl rendering/threading mode in Processing received some major changes in version 0148. These changes are incompatible with old releases of GLGraphics, but this is solved in the latest release, 0.8.2. This release adds a new class, GLGraphicsOffScreen, which implements off-screen rendering to texture. The downloads are available here.
Another advantage of 0.8.2 is that there is no more need of installing a hacked version of opengl.jar, so things like applets and exported applications that use glgraphics should work fine.