More viewer settings

Builds upon !39 (merged)

• Added identity make_renderable, allowing for view(any_renderable)
• Added options: no_outline, no_ssao, and ssao_power
• Fixed crash on double MMB
• Added option to globally register fonts for use with glow-extras-vector text rendering
  • Each ViewerRenderer loads all registered fonts into its own VectorRenderer
  • viewer::global_add_vec2d_font(name, path) and global_clear_vec2d_fonts()