Added back plutovg as a submodule, added option to use system version

[WerWolv]

This PR adds plutovg as a git submodule again and adds the new LUNASVG_USE_SYSTEM_PLUTOVGcmake option to allow using a system-installed version of plutovg instead if needed.

Closes #207

[sammycage]

Thanks for the pull request! Why did you choose to use the LUNASVG_USE_SYSTEM_PLUTOVG option?

option(LUNASVG_USE_SYSTEM_PLUTOVG "Use installed version of plutovg instead of bundled submodule" OFF)

if (LUNASVG_USE_SYSTEM_PLUTOVG)
    find_package(plutovg 0.0.4 REQUIRED)
else()
    add_subdirectory(third_party/plutovg)
endif()

Instead of this?

find_package(plutovg 0.0.4 QUIET)
if(NOT plutovg_FOUND)
    add_subdirectory(third_party/plutovg)
endif()

[WerWolv]

I simply like being explicit. If you rather have it without the extra option, I can remove it

[sammycage]

I get the idea of being explicit, but I think removing the LUNASVG_USE_SYSTEM_PLUTOVG option might actually be better. From my experience, users often don’t notice or know about these kinds of options, so they might not even use it. Plus, adding another option just gives users more things to configure, which can make things unnecessarily complicated.

[sammycage]

Also, could you move the plutovg submodule from third_party to the root directory since it’s the only dependency?

[WerWolv]

Implemented both changes 👍