Introduce the initial Windows backend integration support

[MouriNaruto]

I think we should add Visual Studio (not Code) CMake adaption to this repo as my reply of lvgl/lv_port_pc_visual_studio#114.

This is the first step to achieve this goal.

Maybe the future steps we need:

• Have a CMakePresets.json for common CMake targets which supported by this repo.
• Do some further improvement for lv_conf related things to reduce the manually modifications for lv_conf.

Kenji Mouri

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Summary by cubic

Adds a native Windows backend so LVGL can run without SDL in Visual Studio CMake builds. This is the first step toward full Windows support.

• New Features

  • Windows HAL: adds win_hal_init to create a Windows display and set up pointer, keypad, and encoder input devices.
  • main.c: conditional init for SDL vs Windows; uses lv_delay_ms on MSVC when Windows backend is active.
  • CMake: new LV_USE_SDL option (default ON); conditionally finds SDL and sets draw-unit defines; adds Windows console subsystem and MSVC /utf-8 compiler flags; guards non-MSVC warning options.
  • README: adds instructions to enable the Windows backend.

• Migration

  • Disable SDL: cmake -B build -DLV_USE_SDL=OFF
  • Update lv_conf.h:
    • LV_USE_OS to LV_OS_WINDOWS
    • LV_USE_SDL to 0
    • LV_USE_WINDOWS to 1

^{Written for commit 93fb55f. Summary will update on new commits.}

[cubic-dev-ai]

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