Fix return type of C2D_Fade and C2D_SetTintMode

[KonPet]

Fix returning a bool from a void function

[fincs]

Either remove the return value, or add return true; in the success case

[KonPet]

How did this even compile in the first place though? I mean citro2d gets compiled with -Wall and -Werror. How did it not catch the fact that a bool is being returned from a void function? oreo639 also found out that removing -save-temps makes the compiler give you an error there (can verify that by the way). Does the -save-temps flag remove the -Wreturn-type flag?

[fincs]

I'm equally puzzled.