We need to know the format for other drawables too (like for instance
the native format of a window), so we're pushing this down.

This changes a bunch of references to be RedDrawable::, but not all.
The the old RedPixmap:: references still work, but will be phased out.

We now support 16bit format pixmaps as well as the old ones. Including
both 555 and 565 modes.

We drop the palette argument for pixmap construction as it was only
used for black/white anyway.

Canvas creation is simplified so that there is no separate set_mode
state. Canvases are already created in the right mode and never change.