subed-waveform: add defcustoms for setting height/width of waveforms

[rodrigomorales1]

Increasing the dimensions of the waveforms helps me to quickly analyze the content of the waveform of the subtitle being edited, so this pull request introduces two variables which allow setting the height and width for the image that display the waveforms.

The two pairs of screenshots and code blocks below show how different configurations can set the size for the image that display the waveforms.

(setq subed-waveform-image-width 545)
(setq subed-waveform-image-height 34)

(setq subed-waveform-image-width 800)
(setq subed-waveform-image-height 100)

If you are a GNU/Linux user, you can use the command below to download a sample video with a sample subtitle from Wikimedia Commons.

rm -rf /tmp/subtitles \
  && mkdir /tmp/subtitles \
  && curl \
       --output /tmp/subtitles/video.ogv \
       'https://upload.wikimedia.org/wikipedia/commons/4/41/Creative_Commons_and_Commerce.ogv' \
  && curl \
       --data-urlencode 'format=json' \
       --data-urlencode 'action=query' \
       --data-urlencode 'prop=revisions' \
       --data-urlencode 'rvprop=content' \
       --data-urlencode 'rvslots=*' \
       --data-urlencode 'formatversion=2' \
       --data-urlencode 'titles=TimedText:Creative_Commons_and_Commerce.ogv.en.srt' \
       'https://commons.wikimedia.org/w/api.php' | jq --raw-output '.query.pages.[].revisions.[].slots.main.content' > /tmp/subtitles/video.en.srt

You can preview my changes by cloning the repository (first code block below), save the second code block below to /tmp/subtitles/init.el and use the commands in the third and fourth code block to launch Emacs.

git clone --depth 1 'https://github.com/rodrigomorales1/subed' /tmp/rodrigomorales1-subed

(add-to-list 'load-path "/tmp/rodrigomorales1-subed/subed")
(require 'subed-srt)
(require 'subed-waveform)
(add-hook 'subed-mode-hook 'subed-waveform-minor-mode)
(add-hook 'subed-mode-hook 'subed-enable-sync-player-to-point)
(add-hook 'subed-mode-hook 'subed-enable-loop-over-current-subtitle)
(add-hook 'subed-mode-hook 'subed-enable-replay-adjusted-subtitle)

emacs \
  --quick \
  --load /tmp/subtitles/init.el \
  --eval "(setq subed-waveform-image-width 545)" \
  --eval "(setq subed-waveform-image-height 34)" \
  --eval "(find-file \"/tmp/subtitles/video.en.srt\")"

emacs \
  --quick \
  --load /tmp/subtitles/init.el \
  --eval "(setq subed-waveform-image-width 800)" \
  --eval "(setq subed-waveform-image-height 100)" \
  --eval "(find-file \"/tmp/subtitles/video.en.srt\")"