[bdbarnett]

Fixes #46. #48 has merged, so this is based on main.

On CPython, a struct method is now bound when its first argument is the struct by value, as on MicroPython. Before, it was bound only when that argument was a pointer to the struct. So lv_color_brightness(lv_color_t c) and the rest were generated but never reachable, as methods or as module functions.

>>> c = lv.color_hex(0x336699)
>>> c.brightness(), hex(c.to_u32() & 0xFFFFFF), c.to_32(255).alpha
(108, '0x336699', 255)

All 21 methods listed in #46 are now bound on CPython. A by-value method gets a copy of its struct, so calling it never changes the struct.

area_t.diff was the other case: its first parameter is an array (lv_area_t res_p[]). It's now bound the way MicroPython binds it, as a static method that takes all three arguments.

The MicroPython and CircuitPython outputs are unchanged.

The new smoke test test_struct_methods_by_value calls to_u32, eq, brightness, lighten and to_32, and checks that the other methods exist. Before this change, CPython failed it with 'lvgl.lv_color_t' object has no attribute 'to_u32', and MicroPython passed it. With this change it passes on both locally, along with the rest of the smoke test and lvgl-python's unittests. pytest (109) passes and regenerate_all.sh --check matches.