Replace forward declaration of DrawParams with actual declaration

[edelmanjm]

Image uses img::DrawParams() as as default argument; however, the constructor for DrawParams isn't available until the full declaration is available (see https://stackoverflow.com/questions/16857425/forward-declaration-of-class-in-c-incomplete-type). This caused an "invalid use of incomplete type 'img::DrawParams'" error with clang. This resolves that issue.

[skramm]

Thks for your interest, I'll have a look ASAP.
But FWIW, code is currently heavily refactored: removal of class Root, previously needed for run-time polymorphism, and replacement with std::variant. And switch to C+17 required, that will enable constexpress if.
But shouldnt impact your PR.
And yes, I remember I recently added the default value to the draw function, but forgot to add the relevant test...

[edelmanjm]

Okay cool, happy to use your refactored version once it's merged; just figured it'd be good to ensure that the master branch can at least build in the meantime, since I don't see a release with svg support yet.