feat(color): added option to overwrite lv_color_mix

[domologic]

Description of the feature or fix

A clear and concise description of what the bug or new feature is.

Checkpoints

• Run code-format.py from the scripts folder. astyle needs to be installed.
• Update the Documentation if needed
• Add Examples if relevant.
• Add Tests if applicable.
• If you added new options to lv_conf_template.h run lv_conf_internal_gen.py and update Kconfig.

Be sure the following conventions are followed:

• Follow the Styling guide
• Prefer enums instead of macros. If inevitable to use defines export them with LV_EXPORT_CONST_INT(defined_value) right after the define.
• In function arguments prefer type name[] declaration for array parameters instead of type * name
• Use typed pointers instead of void * pointers
• Do not malloc into a static or global variables. Instead declare the variable in LV_ITERATE_ROOTS list in lv_gc.h and mark the variable with GC_ROOT(variable) when it's used. See a detaild description here.
• Widget constructor must follow the lv_<widget_name>_create(lv_obj_t * parent) pattern.
• Widget members function must start with lv_<modul_name> and should receive lv_obj_t * as first argument which is a pointer to widget object itself.
• structs should be used via an API and not modified directly via their elements.
• struct APIs should follow the widgets' conventions. That is to receive a pointer to the struct as the first argument, and the prefix of the struct name should be used as the prefix of the function name too (e.g. lv_disp_set_default(lv_disp_t * disp))
• Functions and structs which are not part of the public API must begin with underscore in order to mark them as "private".
• Arguments must be named in H files too.
• To register and use callbacks one of the followings needs to be followed (see a detaild description here):
  • For both the registration function and the callback pass a pointer to a struct as the first argument. The struct must contain void * user_data field.
  • The last argument of the registration function must be void * user_data and the same user_data needs to be passed as the last argument of the callback.
  • Callback types not following these conventions should end with xcb_t.

[kisvegabor]

Thank you!