Update PlatformIO demo to LVGL9.1

[zmeiresearch]

This pull request updates lv_platformio demo to use lvgl 9.1. The changes that are required are:

• Bump lvgl version from 8.2 to 9.1
• Remove dependency of lv_drivers. Lvgl 9+ has these included in the main repo under src/drivers
• Simplify the SDL demo as the driver in 9+ provides lv_sdl_window_create et.al. which takes care of proper creation and initialization of a window and input devices
• Update the demo for the lvgl 8->9 transition (e.g. lv_disp_drv_t -> lv_display_t, lv_indev_drv_t -> lv_indev_t)

NB!: SDL demo has been tested only on MacOS and Linux. I've also updated the STM Discovery demo to compile and very similar code works on other boards, but I don't have that particular board to test.

[kisvegabor]

Awesome, thank you for this PR. Could you take a look at the failing CI? It seem on Mac it needs simply SDL.h.

[zmeiresearch]

Pushed an update that fixed the build failure. Looking at previous CI runs, on x64 MacOS versions PlatformIO somehow managed to find SDL.h and libSDL2.a by itself, but on latest one it did not.

Initially tried adjusting build_flags to conditionally search for the correct paths (similar to what is done for hal/sdl2) but it ended up too convoluted, so used an additional extra_script that does the magic.

Note that it needs to be a 'pre' script, as build_flags cannot be modified by 'post' one - see https://docs.platformio.org/en/latest/scripting/launch_types.html

[kisvegabor]

Awesome! Thank you very much!

I've shared it on LinkedIn too.

[Linjieqiang]

Thank you very much. I tried many times and finally succeeded this time.