[Linux] Fix a race between the Flutter renderer vs. FlGlArea size alloc

[jpnurmi]

TODO

Fixes: ubuntu-flutter-community/software#868

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[jpnurmi]

I thought so but I had to try anyway :)

�[0;33mNote: Google Test filter = FlViewTest.SizeAllocate
�[m�[0;32m[==========] �[mRunning 1 test from 1 test suite.
�[0;32m[----------] �[mGlobal test environment set-up.
�[0;32m[----------] �[m1 test from FlViewTest
Unable to init server: Could not connect: Connection refused

(flutter_linux_unittests:8415): Gtk-WARNING **: 09:55:03.940: cannot open display:
[FATAL:flutter/fml/thread_local.cc(32)] Check failed: false. pthread_setspecific failed (22): Invalid argument
[ERROR:flutter/fml/backtrace.cc(108)] Caught signal SIGABRT during program execution.

[robert-ancell]

I think the gtk_widget_size_allocate should only normally be called inside GtkWidget::size-allocate. If the size needs recalculating then gtk_widget_queue_resize should be here? Though I'd expect the gtk_widget_set_parent would trigger that. Perhaps the queue_resize is missing somewhere else?

[jpnurmi]

• Use a single FlGLArea #39473