Surface creation now specifies the exact format, not only the bit depth
of each surface which is used for rendering.
Additionally we now actually store the surfaces in that format, instead
of converting everything to 32bpp when drawing or e.g. handling palettes.
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.
