Remove Region and implement "ProcessInsideShape" within ImageSharp.Drawing

[antonfirsov]

We want to enable clipping interest region of arbitrary processors by an arbitrary polygon. If we agree that solving this problem with Region is not feasible, we need to remove that type for 1.0, and add the ProcessInsideShape utility to the core library.

For more details, see the discussion starting with #96 (comment).

[JimBobSquarePants]

This means removing (not refactoring) any current extensions utilizing Region yes?

• ClearRegionExtensions
• FillRegionExtensions

We'd then keep ShapeRegion as an internal type to use in FillRegionProcessor

[antonfirsov]

Yes, we should remove them, just let's make sure we have equivalent functionality in other extension classes using IPath.