Zero the height and width of the WebGL canvases on cleanup

[Snuffleupagus]

The same idea as PRs #4920 and #4959, but for the temporary canvases used in WebGL assisted rendering.

[CodingFabian]

to assist reviewing: trivial, looks good to me

[timvandermeij]

/botio test

[pdfjsbot]

From: Bot.io (Linux)

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Received

Command cmd_test from @timvandermeij received. Current queue size: 0

Live output at: http://107.21.233.14:8877/19b6c1392e8cc27/output.txt

[pdfjsbot]

From: Bot.io (Windows)

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Received

Command cmd_test from @timvandermeij received. Current queue size: 0

Live output at: http://107.22.172.223:8877/b57f12b31a66ae7/output.txt

[Snuffleupagus]

I would be surprised if there are any test failures here, given that WebGL is off by default and hence isn't used on the test bots ;-)

[pdfjsbot]

From: Bot.io (Linux)

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Success

Full output at http://107.21.233.14:8877/19b6c1392e8cc27/output.txt

Total script time: 37.50 mins

• Font tests: Passed
• Unit tests: Passed
• Regression tests: Passed

[timvandermeij]

@Snuffleupagus Me too, but I wouldn't want to risk merging this without having tested it first (I already tested it locally) :-)

But this is looking good, so it can be merged. Thank you for the patch!