slideshow: log timing info
racket/gui cocoa: use CGLayer for canvas bitmaps

The backing bitmap for a canvas is created as a CGLayer, which can
make screen refresh much faster. Also, calling the `make-bitmap` method
of `canvas%` produces a CGLayer-based bitmap, so drawing those bitmaps
to a fast context can be fast.

The improvement particularly helps with Retina displays, making
DrRacket more responsive and increasing the potential frame rate
of Slideshow.
slideshow: add pict->pre-render-pict

Uses `make-bitmap` from the viewer's canvas to optimize drawing.