Collected trivial bug-fixes

Turns out we actually can't optimise out the second texture normal
lookup - it makes a noticeable difference in-game. Also there was
possible slice collision, it might be worth warning about this kind
of stuff.

Also removed unused variable, and made error messages more consistent
in terms of whitespace.
Framebuffer update improvements

Now we use the shader to do a proper blend - this avoids some artefacts
that were visible before. Also after some hair-pulling the coordinate
transformation now seems to be more correct than before. Maybe even
with zoom. We'll see.