don't attempt to clip layers anymore using glScissor

this seems to hurt performance on some GPU. this change
might negatively affect performance on other GPUs though, but
probably in less time-sensitive cases. If this becomes a
problem it might become necessary to pre-clip the geometry
(so that we don't have to use glScissor).

This improves the rotation animation quite a bit.

Change-Id: I5dbe1286f7ad858ef2c1e1ad9a07ee3f26c0b1f3