Disable stuff that depends on 2dgraphics while we refactor it.

All of this stuff needs to get rewritten and re-enabled once the
refactoring is complete.
Define the new capability interfaces, and test them.

The new design principle is to separate the in-memory storage of an image
(Bitmap2d) from the capability of displaying it. This allows greater
flexibility in how image data is buffered before being displayed; for
example, the PaletteBitmap2d class allows an in-memory bitmap using
indexed color that can then be backed by a true-color display, doing
the conversion on the fly using the palette during display update.
Rewrite the 1D graphics capability to work like the new 2D one.

The 1D graphics capability is just an altered version of the 2D one, so
refactor it along the same lines as the 2D refactoring, including a new
1D-as-2D adapter supporting the new interface.

Lost all of the doc comments in this rewrite; will add them back in a
later commit.
Rework the LPD8806 driver to support the new 1D graphics capability interface.

Functionality is basically the same as before but now the display driver
has no intrinsic display buffer so various different buffer bitmap
implementations are now possible.

Also includes some re-jiggering of the way colors are modeled to make
it easier to instantiate colors separately from instantiating the display
driver, and to make the function constexpr so it's easier to use
in creating statically-allocated color palettes.
Port the SDL implementation of 2dgraphics to the refactored interface.
Refactor the 2D graphics interface to separate bitmaps from displays.