Consider to split `AppData::m_appData` into platform and renderer depended

[ChemistAion]

Such as: https://github.com/ocornut/imgui/blob/6b2e03c5b1bc4f99dbd86d10e5fd12af9e3fe1c2/imgui.h#L2005-L2009

Premise for that:

• all platform stuff, e.g: im3d_impl_win32 internals goes to (as-is): AppData::m_appData
• all renderer depended stuff, e.g.: im3d_impl_dx12 goes to: AppData::m_rendererData <-- TODO

Now, we have a nice AppData for different renderers.... 😇