Shaders: Hot reload, togglable by lua debug command

See merge request OpenMW/openmw!2238

(cherry picked from commit 4078f19c)

8d194a16 Shaders: rudimentary hot reloader on shaders
4e7c1c5b Added break when the operation failed
6b38d622 Added lua bindings to toggle hot reload (disabled by default) or to trigger a reload
31d41252 forgot memory include
f78fa989 fixed include, cleaned comments and indentation
fc8838c7 Renamed lua binding, and use action to avoid concurrency issue
aa51d6de Missing chrono include ?
68d06989 Fixed cyclical included check
b6d7293a Removed weird lines that I thought were necessary to please the compiler
9a475b0c fixed blank lines and missing breaks
cdd95f78 replaced empty function body by default
a1c8dc9d C++17 compat ?
7b78bf4b Fix files with different defines weren't added to the hot reload manager
cc9d4364 includes now work when the same shader has different defines
15751c57 Lua debug api doc
3ab0a991 Hot reload done only once every 200 ms, no point in beeing faster
df69fc76 Post processing shaders now use the same lua commands, no more launcher option...
c71f3508 changed overview.rst of post processing
603b30e1 Added some variable names to make it clearer what their function was
baadc06e Merge branch 'master' into 'SHADER_HOT_RELOAD'
decfbc53 Fix threading issues
b14cc673 adds missing decleration
16a4b571 adds missing include
166717d6 Makes sure threads are only stopped once ,and that they will be re-started
25c1f0ca Renamed variable to fix case issue