fix a memory leak

[claudeha]

Memory usage of improviz would increase linearly over
time until crash from memory exhaustion.

deepseq is a bit of a sledgehammer,
probably a more precise intervention is possible.

The key change is the ... $! force newGe
which makes the data in the TVar be stored strictly.

Otherwise (conjecture) the data refers to previous
copies of the data structures, etc, which leaks
space because the whole data structure must be
retained.

[rumblesan]

@claudeha you are a total wizard haha. just looking at this now and running my own very basic tests, and it's working beautifully. missing the deepseq dependency for the tests but I'll add that separately and just merge this now.

many thanks, this is really appreciated

[claudeha]

I think this was a case of https://wiki.haskell.org/Memory_leak#Keeping_not_needed_references_alive What I did was first run with the basic profiling support when built with stack without special options:     stack exec --rts-options -hT improviz and converted output improviz.hp heap profile to svg with:     hp2pretty improviz.hp (you can also use hp2ps which comes with ghc) which shows which constructors/types take up memory.   This hinted at `Gfx.Engine.SavableState` being part of the problem. To get more information, I rebuilt with profiling:     stack build --profile --executable-profiling --library-profiling     stack exec --profile --rts-options -h improviz     hp2pretty improviz.hp and that showed where in the code the memory was coming from, mainly `popScope` and `pushScope` and matrix stack stuff. The code I was running inside improviz had a couple of nested loops which I guess might have magnified any leak from push/pop. Linear memory increase over time when doing animation typically indicates previous frame data not being freed, which can happen if you refer to part of a structure lazily (retaining the whole thing) instead of making a copy of only the part you need.  One solution is to `force` the data structure with `deepseq` package, it's a bit heavyweight because it traverses the whole structure each time instead of just the parts that are needed.  This in conjunction with `$!` when writing to the variable to ensure the forcing actually happens promptly.

[rumblesan]

aahh, amazing! was planning to email you to ask for some info on how you did the profiling. this is gold, thankyou.

I was doing some testing today, and one of the things I realised after having had it running for half an hour was my laptop fan hadn't kicked in yet hahaha