Stack allocator memory optimization

[nihiL7331]

This PR implements a stack allocator (linear allocator with rewinding capabilities), used to optimize scenarios where temporary heap memory is necessary.
The scratch is used throughout the whole kitty graphics protocol logic, in OSC52 and during font baking when oversample is turned on.

Changes:

• Due to the usage of _Static_assert and _Alignof, the C standard was bumped from C99 to C11.
• Because of the structure of the codebase, there's really not a reason to not keep functions not inline. That's why all of the functions got the inline keyword. That's because every function is either called once or is tiny enough, where the compiler inlining the function doesn't affect the binary size at all.
  -While rewriting kitty logic, certain parts got optimized:
  • cropping is now in-place,
  • scaled image is placed on top of the original image,
  • all memory is freed just when its not needed during the kitty render loop.
• Sixel got a small optimization, where on commit there's no additional allocation, instead using the same in-place trick as in image scaling in kitty.
• Changed the default SFTE_TERM_ALT_SCREEN value back to 1.