Release 0.9.5 "Everlasting" - Power management, offline graphics

This release brings two major new features: vastly reduced power
drain, and off-line persistent graphics.

New features
============

* Off-line graphics are the ability to draw images in memory instead
  of directly on screen. Like on HP calculators, this works by storing
  a graphic object in `PICT`. Unlike HP calculators, `PICT` is an
  actual variable in the current directory, meaning that you can have
  multiple `PICT`, although bitmaps use a lot of memory. For example,
  running `500 300 BLANK 'PICT' STO` will create a 500x300 pixels
  off-line image in `PICT`, and all graphic commands after that will
  draw in that off-line image instead of on screen. Off-line graphics
  are not immediately shown on screen, but are displayed before
  returning to the command line in a way similar to the `Show` command
  (i.e. centererd if smaller than the screen, and with scrolling if
  larger). They are also persistent, meaning that you need to
  explicitly `ClLCD` to erase them.
* Add `Blank`, `BlankGrob`, `BlankBitmap` and `BlankPixmap` commands
  to create a blank image with default format, in HP GROB format, in
  DB48x bitmap (Black and white) format and in DB50x pixmap format
  (color RPL only).
* Add `→HPGrob`, `→Bitmap`, `→Pixmap` commands which can be used to
  generate an HP-compatible GROB format, a bitmap or a pixmap (color
  RPL only) from an object. If the object is already a graphic,
  perform conversion. Note that on color RPL, conversions between
  color and monochrome formats are supported.  Colors are converted to
  monochrome using grayscale patterns.
* Add `→LCD` and `LCD→` commands to send an object to the current
  graphics, or to create a graphic object from the current screen
  content.
* The `Off` command can now be used in a program and will preserve
  what is on screen. You can resume execution of the program by using
  the `ON` key.  Add `PowerOffWithImage` command to show off-images
  even from programs.

Bug fixes
=========

* Fix display of object `Info` in the interactive stack
* Clear system timers before entering system menu, so that the self
  test menu entry does not run a fast-paced loop.
* Fix off-by-one error in result of `LastBitSet` (number bits starting
  at 0)
* Correctly render color pixmaps when on the stack (color RPL only)

Improvements
============

* Reduce power usage dramatically by disabling timers when going to
  sleep.  This manifests as a higher battery voltage being shown,
  which is closer to what the DM42 firmware or integrated self-test
  would show. This will most likely improve battery life for
  interactive uses of DB48x.
* Reorganize the `MemoryMenu` to make both GC and runtime statistics
  available
* Reorganize the `GraphicsMenu` by topics to accomodate the new
  commands
* Add image checks in the test infrastructure for RPL code examples
* Add tests for RGB colors
* Repair `make compare` which was trying to build a binary from
  `compare.cc`
* Make `Sub` a compatibility spelling for `Extract`, not the default
  one
* Adjust low battery indications to avoid premature warnings
* Make `debug_printf` use lower-level routines that can run earlier
  after boot
* Add `make INSTALL_PGM_ONLY=y install` option for faster install
* Generate DB48x bitmaps by default instead of HP-compatible
  GROBs. Bitmaps use less memory when the width is not a multiple of
  8.
* Make test of pixel functions more reliable by increasing their timeout
* Add help for `ModesMenu`, `SymbolicResults` and `NumericalResults`

Signed-off-by: Christophe de Dinechin <christophe@dinechin.org>