Trim down meson.build and meson_options.txt

[WillPower3309]

meson.build is now mostly the default from wlroots. Since scenefx is just a subset of wlroots, we should be able to trim down a lot of the meson.build file.

[swwwee]

#7 Since vulkan renderer is not in yet, I think it should be removed from meson_options for now. It caused confusion (at least for me) when I was version bumping swayfx to 0.4 on Gentoo GURU. Vulkan is currently a USE flag (i.e. compile in vulkan support) for the scenefx package so it reflects the meson build/options, but I'm probably going to remove it downstream.