(love 11.0) Use normalized colors; fix broken sketch and filmgrain

[idbrii]

Update to love 11.0 colour format where all colours are normalized in [0,1].

Also fix some invalid indexing.

While some effects will continue to work so long as users pass 255 color values (desaturate, scanlines, vignette), the others are broken in love 11.0 without this change (sketch, filmgrain).

This change would break behaviour for use with love 0.10. Not sure how you want to deal with that. Maybe make a pre-11.0 tag?

sketch

Completely broken -- nearly no pixels modified.

filmgrain

Very broken -- few grains visible.

Test

I flipped through the effects in the demo and they all seem to work now (or at least have some effect).