src/gfxcore.cc,src/gfxcore.h: Reimplement animation so that it's
based on angular change per unit of elapsed time, rather than
averaging the time take for the last two scene redraws. This gives
a more even animation in the face of variable load and scene redraw
time, and should be more consistent between platforms.
src/gfxcore.cc: Switch to a point of the compass during auto-rotation
now jumps straight there rather than the two animations fighting.
src/gfxcore.cc: Reduce the maximum auto-rotation speed, as the
previous limit was uselessly fast.
