Private gfx type RenderContext is exposed in several public APIs

[jdm]

RenderContext is a public member of the private render_context module. It's used as an argument for Font::draw_text_into_context and display_list::draw_into_context, which are public methods used in other gfx modules. The proper way to solve this is via public traits in private modules like render_context, I guess, so the type doesn't leak into publically-exported types and interfaces.