If do_scale ever failed, it would result in graphics buffers not
returning to the pool, resulting in them never being reused.

People are used to thinking about the aspect ratio of displays and
used to not thinking at all about that of pixels.  Most of the modes
are simplt 4:3 modes and the text area is the entire window.  The
only oddball here is the Commodore 64 and 128 40-column modes.
Because they have a border around them that's wider on the sides than
the top/bottom, the display aspect ratio is actually narrower than
a normal NTSC screen (6:5).  It seems the PAL version actually has
square pixels, but nobody has asked for a PAL Commodore mode, and I
think that has a different colour palette too so I'm not doing it.

Just to frustrate DigitalMan a bit, the default custom aspect ratio
is now 4:3 (but can be configured).  At present, modifying the custom
mode while *in* the custom modes "works", which no sane person would
want when adjusting the aspect ratio.

#pragma clang causes an error
__attribute__ not supported

This uses the 720x400 with 9x16 character cell that always made
BBSs look so terrible.

Simplest way is to use the alt-left/right snapping.

Which required a new ciolib option of course.
Only usable in X11 mode at present, though it may land for SDL
this weekend.

Also, we don't need to use pointers for the scaling.

Since we have all odd numbers thanks to the pointy scaler, and we
have 2x thanks to xBR, we should now be able to build any integer
scaling from a combination of those two scalers.

This, of course, assumes you have enough CPU and RAM to actually
do the scaling you want... powers of two will likely be the worst
case for scaling.

Also, do pointy scaling before xBR.

SyncTERM doesn't support any "fat-pixel" modes.

1) Initialize the r2y array for xBR so it actually works.
2) Add a vertical (only) interpolation scaler for aspect ratio enforcement
3) Add a simple muliplier scaler, so that can be removed from x_event.c
4) Use a new graphics buffer free list, which allows tracking last
   drawn screen instead of last bitmap rectangle, removing various hacks
5) Share the Y'CbCr <-> R'dG'dB'd tables between xBR and scale.c

Uses "pointyscale" for x3 and x5
Uses xBR from FFmpeg for x2 and x4
HQx is also included, but unused as it's too slow at x4.