Fixes for a whole bunch of warnings

These warnings were always enabled, but we didn't see them due to https://gitlab.com/OpenMW/openmw/-/issues/7882.
I do not fully understand the cause of 7822 as I can't repro it in a minimal CMake project.

Some of these fixes are thought through.
Some are sensible best guesses.
Some are kind of a stab in the dark as I don't know whether there was a
possible bug the warning was telling me about that I've done nothing to
help by introducing a static_cast.

Nearly all of these warnings were about some kind of narrowing
conversion, so I'm not sure why they weren't firing with GCC and Clang,
which have -Wall -Wextra -pedantic set, which should imply -Wnarrowing,
and they can't have been affected by 7882.

There were also some warnings being triggered from Boost code.
The vast majority of library headers that do questionable things weren't
firing warnings off, but for some reason, /external:I wasn't putting
these Boost headers into external mode.

We need these warnings dealt with one way or another so we can switch
the default Windows CI from MSBuild (which doesn't do ccache) to Ninja
(which does).
I have the necessary magic for that on a branch, but the branch won't
build because of these warnings.

components/lua/scriptscontainer.cpp

@@ -590,7 +590,7 @@ namespace LuaUtil
         updateTimerQueue(mGameTimersQueue, gameTime);
     }
 
-    static constexpr float instructionCountAvgCoef = 1.0 / 30; // averaging over approximately 30 frames
+    static constexpr float instructionCountAvgCoef = 1.0f / 30; // averaging over approximately 30 frames
 
     void ScriptsContainer::statsNextFrame()
     {