Add an abstract readBlock method in the DecodeStream class

[Snuffleupagus]

This avoids having to "duplicate" dummy readBlock methods in a couple of image-stream classes.
Also, move a few DecodeStream field definitions to (ever so slightly) shorten the code.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.97%. Comparing base (08eca52) to head (1f6bfa0).

Files with missing lines	Patch %	Lines
src/core/decode_stream.js	86.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21190      +/-   ##
==========================================
+ Coverage   63.94%   63.97%   +0.02%     
==========================================
  Files         213      213              
  Lines      134597   134595       -2     
==========================================
+ Hits        86064    86103      +39     
+ Misses      48533    48492      -41     

Flag	Coverage Δ
fonttest	8.66% <0.00%> (?)
unittest	56.30% <83.33%> (-0.01%)	⬇️
unittestcli	63.34% <88.88%> (+0.01%)	⬆️

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[timvandermeij]

r=me; thanks!

[Snuffleupagus]

/botio browsertest

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Received

Command cmd_browsertest from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.241.84.105:8877/e04c21e125bd951/output.txt

[moz-tools-bot]

From: Bot.io (Windows)

---

Received

Command cmd_browsertest from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.193.163.58:8877/d21ca3e6a285da5/output.txt

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Failed

Full output at http://54.241.84.105:8877/e04c21e125bd951/output.txt

Total script time: 18.48 mins

• Regression tests: FAILED

  different ref/snapshot: 1

Image differences available at: http://54.241.84.105:8877/e04c21e125bd951/reftest-analyzer.html#web=eq.log

[moz-tools-bot]

From: Bot.io (Windows)

---

Failed

Full output at http://54.193.163.58:8877/d21ca3e6a285da5/output.txt

Total script time: 24.62 mins

• Regression tests: FAILED

  different ref/snapshot: 1

Image differences available at: http://54.193.163.58:8877/d21ca3e6a285da5/reftest-analyzer.html#web=eq.log