improve inference and caching of draw_mesh

[SimonDanisch]

• Make draw_mesh more inferrable and specialize less to crazy mesh type.
• also add some direct calls for drawing the mesh that's not cached because of inference failures.
  Merged refactor and improve conversions #1743 into this PR, since it was improving similar areas and needed the same fixes.
  This includes a few breaking changes:
• breaking to_colormap(...) always returns Vector{RGBAf} now... Still need to contemplate, if that's a good change, since colormaps should almost never contain alpha - but it spilled over from to_color which often contains the alpha so i made sure that to_color always returns Union{RGBAf, Vector{RGBAf}}
• breaking removed conversion for tuples of colors for colormap (so colormap = (colors...,) isn't accepted anymore)
• breaking removed to_colormap(gradient, n), since it was very inconsistent and impossible to actually implement. One should use Makie.categorical_colors(gradient, n) for this.
• removed fallback to_xxxx(value) = value for to_colormap/to_color/etc
• one now overloads to_xxx instead of convert_attribute(value, key"xxx")

[MakieBot]

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