libretro: Add no sprite limit core option

[msheehan79]

This PR implements a No Sprite Limit core option to the Libretro core, to turn off the 4 sprite per line limitation that is present on original hardware which results in less sprite flicker.

A good test case I used was "BurgerTime" as this game introduces some very notable flickering immediately upon starting, and the core option removes this.

I did not add a getter method as the libretro implementation does not utilize one, and I was not sure if you would prefer an inline function or not as some of the other getters use.

[drhelius]

Awesome. Thanks a lot for your work.

Just two things:

• The variable shouldn't be added to the save state as this is just a rendering option. It doesn't effect the emulation so I won't expect it in the state. This change will also make existing states invalid unless you add some logic to let old states work with this new value. I would remove it from the save state system.
• SetNoSpriteLimit() in GearcolecoCore is probably not needed cause you can use core->GetVideo()->SetNoSpriteLimit() but this is just cosmetic.

[msheehan79]

Thanks for the feedback and pointers on these - I will adjust soon.

[msheehan79]

@drhelius I've incorporated your feedback into the PR.

I also had originally had the flag getting set to false in the Reset() method which I had to remove, as I realized that method gets called after Libretro sets the flag, so it would keep getting reset until you went back into Core Options and toggled it off and on again.

[drhelius]

Nice! Thanks again.