[gui] Fix flutter rendering problems

[tobe2098]

This PR adds an environment variable to do proper software rendering within the flutter GUI. This avoids flickering problems when flutter attempts to send GPU instructions to the CPU in Mesa drivers (or possibly when drivers are not available) according to flutter/flutter#169508. Fixes #4369 (tested) when this PR is merged into main.

This solution should only be temporary until we (or flutter) solve the root cause of bad rendering, since it could impose some additional load in less modern CPUs.

Additional info

• It seems like it is only caused by snap confinement, since the built version has no problems.
• Previous versions of flutter do not enable this fix, probably due to bad flutter software rendering, but the flickering is the same. So, it requires [submodules] Update flutter to 3.35.7 #4441
• This problem appears after the latest 1.16.1, unclear what caused it in our end. It could be a flutter update, since it seems to have been their problem. I examined the diff between main and the 1.16.1 tag and there was nothing obvious that could have caused it (no driver change, no snapcraft.yaml change or linux.patch change).

MULTI-2357

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.46%. Comparing base (b293e8d) to head (9dc6c20).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4475   +/-   ##
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  Coverage   89.46%   89.46%           
=======================================
  Files         240      240           
  Lines       13829    13829           
=======================================
  Hits        12372    12372           
  Misses       1457     1457           

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[tobe2098]

For future reference when the flickering issue is revisited, the following tool can be used for debugging: https://renderdoc.org/.

[tobe2098]

The fix works, the flickering is gone in this PR's snap. Only issue is the following warning message and possible CPU consumption:

** (multipass_gui:9796): WARNING **: 11:15:58.947: Using the software renderer. Not all features are supported. This is not recommended.

To switch back to the default renderer, unset the FLUTTER_LINUX_RENDERER environment variable.

Which appears in the GUI's logs. For this reason I would like somewhat additional testing before approval, hopefully in less modern machines.