Improved Sixel height and width, on_resize.py improved

[jquast]

Improve sixel size order-preference by suggested implementation from @j4james,
pexpect/ptyprocess#79 (comment)

Pixel/Window and Sixel sizes are different because there is almost always some "remainder" in the margins of a window:

Sixel cannot draw in the black margin areas (smaller).

on-resize.py

The example script and surrounding documentation was improved to make this distinction clear, and, design tips on how "safely" use signal handlers as well as "de-bouncing" in our example.

Some example outputs of bin/on_resize.py:

• Konsole height=29, width=143, pixel_height=764, pixel_width=1574, sixel_height=754, sixel_width=1573
• Bobcat height=23, width=115, pixel_height=0, pixel_width=0, sixel_height=529, sixel_width=1150
• kitty height=44, width=160, pixel_height=880, pixel_width=1440, sixel_height=0, sixel_width=0 (no sixel)
• ghostty height=24, width=119, pixel_height=528, pixel_width=1190, sixel_height=0, sixel_width=0 (no sixel)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.29%. Comparing base (496071b) to head (23d8d3f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #320   +/-   ##
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  Coverage   98.29%   98.29%           
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  Files          11       11           
  Lines        2460     2470   +10     
  Branches      423      428    +5     
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+ Hits         2418     2428   +10     
  Misses         32       32           
  Partials       10       10           

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