feat(draw_sw): implemented radial gradient background

[zjanosy]

Description of the feature or fix

Implemented radial gradient background. Work in progress. For internal testing only.

Notes

Added example lv_example_styles_16.c. To test you need to add
#define LV_DRAW_SW_COMPLEX_GRADIENTS 1
to lv_conf.h, and add
extern "C" { void lv_example_style_16(void); }
to main.

[kisvegabor]

Thank you, just a few minor comments at first look.

[kisvegabor]

Suggested change

	// Integer Square Root function
	Integer Square Root function

[kisvegabor]

@zjanosy
It seems incredibly powerful! Please add some tests so that we can also see the whole power of this feature and also fix the CI failures. 🙂

[liamHowatt]

Not done reviewing yet but here are some comments so far.

[liamHowatt]

@kisvegabor, FYI, another instance of CI failing due to a memory error in the RLE decompressor. This time on 32 bit x86. Before it was 64 bit ARM.

[liamHowatt]

Impressive mathematics. If there were any more than the 3 added types of gradient and they also had the same function signatures for setup, get_line, and cleanup, I'd say make a function pointer table/struct, but that only stands to make things less readable and harder to change so I think this is great as-is.