Added "Radius Percentage" setting in TwistEffect

[Lehonti]

For now it's <= 100, but later on we could try to allow percentages beyond 100 and/or moving the center of rotation by using CairoExtensions.GetBilinearSampleClamped, CairoExtensions.GetBilinearSampleWrapped and so on

[cameronwhite]

Going beyond 100% seems reasonable to me if you want to expand the size of the twist effect more

I'm not sure if I'm clear why that implies that bilinear sampling is necessary (although maybe we should be doing that regardless, especially since there are sub pixel samples for antialiasing)
Is that more needed just for the clamping of locations outside the image bounds?

[Lehonti]

@cameronwhite, yes, I am referring to clamping (or reflecting/wrapping/etc.) the locations outside the image bounds.

But there is a caveat when it comes to bilinear sampling: the methods in CairoExtensions.Samples.cs only accept float coordinates, but the calculations inside TwistEffect are in double. We could:

• Wait until the methods have a double version
• Convert the coordinates to float now, and then remove the conversion when the methods have a double version

[cameronwhite]

I'd lean towards just switching those methods over to double entirely - it's odd having them as float when PointD is our standard type for non-integer coordinates

That can be a separate PR in the future, though, so this change seems good to merge