Fixes sixels disappearing with selection issue.

function.  This allows simple fixing of clreol() and clrscrn() which were
not aware of 256-colour mode.

cputs and cputch aren't really needed anymore, but they may be useful, so
leave them in.

adjust the extended fg/bg after the call.

1) The colour values are expressed as a percentage, not a 0-255 value.
2) Implement CSI ? 80 h/l to set/reset the sixel scrolling mode (defaults to
   set).
3) Document sixel commands.
4) Combine the extended set/reset mode commands in the documentation now
   that multiple modes can be set or reset with a single sequence.

XBin (*.xb) files. The display dimensions of the files may be larger (wider and
longer) than the terminal window. You can use the arrow keys/home/end/num-pad
equivalent (including diagnals) to move the display window around.
If you have a recent SyncTERM 1.1b build, you'll get palette redefinitions
and (any version of SyncTERM 1.0 and later) you'll get the custom fonts too.
Note: the entire Bin/XBin file is loaded into JavaScript arrays/objects, so
very large files may require increaisng your JavaScript heap size considerably.

XBimages) and moved the read() method there.

this function for classic .xb files with custom palettes and support
the new SyncTERM/XTerm palette redefinition and reset esc sequences.

Darwin because it's easier, and FreeBSD so I can reproduce problems it
causes on Darwin.

With the supported compilers, we can't rely on the return value of snprintf()
and historically have replaced it with safe_snprintf().  For macOS however,
snprintf() is apparently a macro, not a function, so gendefs.h throws and
error due to redefinition.

Rather than hack around on that, just ensure that we never use the return
value of snprintf() and use safe_snprintf() to ensure we know what the
return value will be.

new colour.

point of the function.

with palette redefinitions when the window scaling changes in SDL Surface mode.

savescreen() tried to copy one byte past the left and bottom edges of the
screen, and getpixels() let it.

Types were wrong for foreground and background data in ciolib_screen.

be what is ment/expected.

struct ciolib_pixels *getpixels(sx, sy, ex, ey)
Returns and allocated array of pixels suitable for use by setpixels()
Must be freed using freepixels()

freepixles(struct ciolib_pixels *)
Frees a struct ciolib_pixels returned by getpixels()

setpixels(sx, sy, ex, ey, x_off, y_off, pixels)
Draws the pixels on the screen. x/y_off are an offset in the pixel array

struct ciolib_screen *savescreen()
Saves the entire current screen state.

freescreen(a)
Frees a value returned by savescreen()

restorescreen(a)
Restores the entire screen state

This allows SyncTERM to not crap all over Sixel graphics.

displaying a graphic centered in the terminal window.

fonts.ini file (e.g. as a logon event or executed via @exec in
answer.asc).

Either run it with no options to demo all the loaded/built-in fonts, or:
load it with a path of some font files (e.g. ctrl/fonts/*.f16) and it'll
demo those font files.

default). See ctrl/fonts.ini for more details.