Added some scrolling graphics optimizations

Added notChanged parameter to shiftThings, shiftBoth, etc. Marked things
as changed in QuadsKeepr::determineThingQuadrants. Added bounds checking
for Quadrants related to a PixelDrawr's MapScreener (added private
variable).
No innerWidth, renamed drawThingOnCanvas -> Context

PixelDrawr is looking good. Still laggy but not as bad as before.
Bounds checking in PixelDrawr::drawThingOnQuadrant