Stack optimizations

[mthiesen]

I have optimized the stack usage of a lot of functions, as we were having issues with stack overflows recently.

These are the functions I have optimized:

Function	Stack usage before	Stack usage after
fp_open_custom()	528	48
setCustomTheme()	1680	424
getCustomTheme()	1696	376
setDisplayOptions()	1288	272
getDisplayOptions()	1272	128
setLedOptions()	1856	248
getLedOptions()	1048	80
setAddonOptions()	1600	240
getAddonOptions()	1104	176
getGamepadOptions()	704	208
setGamepadOptions()	760	128
setSplashImage()	1184	168
setPS4Options()	896	128
setPinMappings()	616	328
addStaticThemes()	864	384
Storage::setDefaultSplashImage()	1032	16
ConfigManager::setSplashImage()	1032	8

This should give us a lot more stack space headroom especially in webconfig mode.

Also, I have enabled PICO_USE_STACK_GUARDS in Debug builds to make it easier to find and diagnose stack overflows.

[arntsonl]

So fresh, so clean!! This is huge for keeping our memory down and adding new features, thank you!