Resolves https://gitlab.com/OpenMW/openmw/-/issues/8100
Also removes some old crud.
Hopefully the old crud is all:
* Handled automatically by CMake now we're using the modern approach.
* A hack-fix for a problem caused by not using the modern approach.
* Massively outdated so no longer necessary.
If it turns out this makes CI fail, I'll tweak things as necessary.
Changes that might not be wanted include:
* Getting rid of our BOOST_STATIC CMake option. In cases where the CMake config doesn't make the one correct choice from the build environment (i.e. because there's a choice) the CMake config exposes the option already.
However, we were forcing this on for Windows, so that might matter.
It seems to default to static on my machine even though I thought I read something suggesting otherwise, so we'll see how things go with that.
If we eventually put CMake in charge of installing dependency DLLs this will be a moot point as we won't need to care.
* Bumping the minimum version of Boost to 1.70.0, as that's the first with working CMake config.
It's from 2019, so plausibly there are distros too scared to use a library from five years ago as it can't legally drink in the US (although it could in limited quantities with parental supervision in the UK, as long as it's just something inconsequential like a single sip of beer).
Size of the files is in order of megabytes at max. Storing offset lookup table
to read from file on demand is less efficient than reading from memory for such
size.
Read and store offsets first. Sort them to read values sequentially. Memoize
last offset and value to avoid reading the same value twice. Use seek only when
current possition does not match offset. Optimize seek for short distance by
calling read instead.
