[lua] FFI function binding

[lampysprites]

Hello! I saw #4123, and had something done about that locally by compiling cffi statically with lua. Which is close to what luajit has - alternative to "luaopen_xxxxx" type of shared libraries, and unlike it, it binds normal dll's without initializing them with lua state. It's still a workaround though, which might end up bothersome to support.

Changes:

• added ffi global to lua, which is the cffi-lua module
• added libbfi and cffi-lua submodules, both of which i patched a bit - cmake file in libffi, and some overloads in cffi-lua that don't pass msvc. (upd: no longer needed)
• added ENABLE_FFI compilation flag

Benefits are pretty obvious:

• compared to loadlibrary, ABI and compiler compatibility becomes irrelevant.
• compared to interpreter, way way faster execution - namely, for anything that requires reading/writing image data.
• allows to do a whole lot of things with only limitation that the application state is not shared.

Downsides:

• no security whatsoever, as pointed out by @ dacap on Provide support for requiring c libraries in scripts #4123:
• loadlib stil won't work, thus neither will already existing lua modules.
• it's harder than scripting - requires some basic understanding of C and memory management. And even though it's copy-paste, it's still bothersome to create bindings for libraries that use transparent pointers.
• crashes are very easy to enjoy in just a few lines of code.
• dependency on libdl on linux - could that affect distribution?

Example/tests:
Source and compiled version: ffi_test.zip - includes minimal example; image modification; using external library (miniaudio)

overall a binding can look like this:

local ffi_test = ffi.load("ffi_test.dll")
ffi.cdef("void grayscale(unsigned char * bytes, unsigned long length);")
local function grayscale(image)
  local bytes = image.bytes
  local sz = image.rowStride * image.height
  ffi_test.grayscale(ffi.cast("unsigned char *", bytes), ffi.cast("unsigned long", sz))
  image.bytes = bytes
end

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I have signed the CLA following the steps given in https://github.com/igarastudio/cla#signing

[aseprite-bot]

clang-tidy review says "All clean, LGTM! 👍"

[lampysprites]

sorry abt the mess - i used wrong patch of cffi-lua; but macos issue is something else and it'll take a bit of looking

[lampysprites]

Issues with compilation fixed, hopefully.
Also figured how to keep build scripts separately (third_party/ffi-cmake) - everything now works with upstream repos without my oonga-boonga patches.
The last remaining addition i can think of is adding a flag for linking to shared libffi, like it is done for some deps. But also it is how it is not done for other deps, so tell me.

[aseprite-bot]

clang-tidy review says "All clean, LGTM! 👍"