Add some fill apis layer in difference mode

[Forairaaaaa]

Hi! I took @tobozo's advices from previous issue to render by line. I try to add 4 apis and run some tests:

• drawFastHLineInDifference
• fillRectInDifference
• fillSmoothRoundRectInDifference
• fillSmoothCircleInDifference

Hope this is useful, thanks~

[tobozo]

awesome effect! thanks for sharing :+1:

some thoughts:

high level drawing functions are better off the LGFX_Base class, although it's useful, very well written and documented, it's unlikely this PR will be accepted.

the reasons are multiple :

• the current trend is to reduce code volume and maintenance in LGFX Base to ease the integration of the upcoming LGFX_HAL
• LGFX_Base should only contain primitives functions, capable of operating seamlessly at any color depth
• special/additional effects built on top of LovyanGFX should not be part of the base class. for example the drawGradient* functions are considered as "intruders" in the Base Class because they only support RGB-based color depths (try to draw a gradient in a sprite with colorDepth 4, 2 or 1 and you'll see what I mean)
• most high level FX functions are project dependant and should be treated as optional, not as default

maybe it's time to create some LovyanGFX-affiliate repository to store community contributions such as yours, I can't make that decision as I'm not the repository owner but it could be worth starting a discussion on that topic

until such thing happens, one way to share your FX code without integrating into the main class is to use inheritance in a separate hpp file that can be optionnally loaded as I did in this example.

class LGFX_SpriteFx : public LGFX_Sprite
{
  public:

    LGFX_SpriteFx(LovyanGFX* parent) : LGFX_Sprite(parent) { };
    /// ...
}

the other "uglier" way to stay outside LGFX_Base and even drop inheritance is to add LovyanGFX* object as first argument to the FX functions and eventually use a cpp template for color types:

  template <typename COLOR>
  void myFXFunction( LovyanGFX* gfx, int32_t x, int32_t y, uint32_t w, uint32_t h, COLOR color )
  {
    assert(gfx);
    // ...
  }

[Forairaaaaa]

I got the idea, thanks again~