Move performance vs quality branching from Compositor to Viewport Pipeline

This will allow for more fine-grained control over performance settings,
without one preference (e.g. viewport quality) overflowing into
unrelated areas (e.g. thumbnail generation, which also uses the central
compositor).

Note that these new user-facing performance preferences do not affect
any of the core compositor optimizations, like dynamically switching
between GDI+/BitBlt/StretchBlt depending on the region being processed.
Performance optimizations like that are *always* active; these
user-facing performance preferences only affect the interpolation method
used when there's no "right" trade-off between quality and performance.
(By default PD errs on the side of quality, but users on low-end PCs may
want to err on the side of speed.)
Add performance preference for Thumbnails

Higher quality image and layer thumbnails = more CPU cycles.  PD tries
to be smart about regenerating thumbnails only when absolutely
necessary, but this is still an easy place for users of low-end PCs to
save some CPU cycles.