Add a feature flag to hide hybrid capability

[AlanGriffiths]

The intent is to allows us to merge without exposing functionality we are still polishing.

This uses an environment variable (MIR_EXPERIMENTAL_HYBRID_GRAPHICS), not a configuration option, as that is easy to drop seamlessly later.

[codecov]

Codecov Report

Merging #3072 (dbef651) into platform-API-merge (839a0aa) will increase coverage by 0.03%.
The diff coverage is 88.88%.

@@                  Coverage Diff                   @@
##           platform-API-merge    #3072      +/-   ##
======================================================
+ Coverage               75.79%   75.82%   +0.03%     
======================================================
  Files                    1057     1057              
  Lines                   73992    73922      -70     
======================================================
- Hits                    56079    56052      -27     
+ Misses                  17913    17870      -43     

Files	Coverage Δ
src/server/graphics/default_configuration.cpp	79.57% <88.88%> (+0.37%)	⬆️

... and 16 files with indirect coverage changes

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[AlanGriffiths]

::   147 |         platform_modules.resize(std::min(1ul, platform_modules.size()));
::       |                                 ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Rats! I wrote "1uz" and the compiler complained that I was using c++23.

[Saviq]

Awesomes.