scenes/opengl: Remove half-pixel correction workaround

It was needed to work around visual glitches in the wobbly windows
effect. Since the wobbly windows effect renders the animated window into
an offscreen texture, we don't need this workaround anymore.

Furthermore, rather than using half-pixel correction, it is more
desirable to use an offscreen texture as it results in simpler design.
Performance-wise, it's not that bad that we need to start looking for
other ways to get rid of the seams between window contents and deco.