Give BlendMode a temporary type.
Fix up mutability on some FFI functions.
Revise Window via 'with_ptr' function.

with_ptr allows access to the internal pointer through an
explicitly unsafe block, instead of openly inviting dangling
pointers to be created.
Add defaults for RenderInfo, Rect and Point.
Add 'with_ptr' methods to Surface.
Add stub Texture object.

Used to satisfy dependency in Renderer object later.
Add mostly-complete wrapper for SDL_Render*
Merge in wrapper for SDL's render functions.