Fix SDL generation for interfaces

[emeryotopalik]

Fixing #977

Per the issue, the SDL renderer was generating:

interface Interface { ... }
type ImplementingType  { ... }

instead of:

interface Interface { ... }
type ImplementingType implements Interface { ... }

[binaryseed]

Hmm, CI seems broken :(

[zachdaniel]

If at all possible, a release with this issue resolved would be a huge help to a project on my end. Thanks for your hard work!