Module support is easy to break from the header side, e.g. by a new
system include, a forward-declared platform type, or an export block
that misses a declaration. This adds a CMake project that builds every
module whose platform library is found, alongside the header build of
the same code.
Linking alone doesn't show that the module behaves like the headers,
so the main check is differential. 'DIFF' tests are compiled once with
'import imgui;' and once with '#include "imgui.h"'. Both builds run
the same scripted frames and must print the same hash of the draw data
and texture uploads (test_diff.h, compare_output.cmake). Backends that
need a GPU or a display are only linked, and wgpu is only compiled.
The core module and the null backend are also built for each
compile-time configuration that CI builds example_null with, because
IMGUI_DISABLE_* and friends change what gets exported. Presets use
Clang, Ninja and 'import std'. Toolchains for cross builds belong in
CMakeUserPresets.json, which is ignored along with build/.
