clean up mesh mess

[SimonDanisch]

The mesh type from GeometryBasics has been pretty insane (in terms of type complexity & usability) and making it the core representation in the mesh plot type has been a mistake for compilation latency and usability reasons.

This refactors the mesh plot type to get rid of GeometryBasics.Mesh, and instead just write out the mesh attributes we need.
We gain the following advantages:

• make it easy to switch to the slimmed version of GeometryBasics Slim down GeometryBasics and remove all type complexity JuliaGeometry/GeometryBasics.jl#173
• enable users to do mplot = mesh!(ax, ..., normals=my_normals) and mplot.normals = new_normals etc...Before that, all these would have been wrapped up in mplot.mesh and it wasn't possible to update them separately
• prepare for a world were the basic plot types are well documented and typed.
  The fully typed mesh type will look something like:

struct MeshPlot{T, N}
    vertex_colors::Union{Nothing, RGBColors}
    image::Union{Nothing, Sampler{T}}
    colormap::Union{Nothing, Vector{RGBAf}}
    colorrange::Union{Nothing, Vec2f}

    normals::Union{Nothing, Vector{Vec3f}}
    texturecoordinates::Union{Nothing, Vector{Vec2f}}
    vertices::Vector{Point{N, Float32}}
    faces::Vector{GLTriangleFace}

    backlight::Float32
    shading::Bool
    fetch_pixel::Bool
    uv_scale::Vec2f
    transparency::Bool
end

And will then very cleanly lead to type stable code in the backends without the current mess, where nobody knows what the mesh plot type is even supposed to contain.

Switching to this representation will need to wait though and will be done in the attributes refactor.

[MakieBot]

Compile Times benchmark

[SimonDanisch]

Closed in favor of #2056