Feature request: contour that can handle curvilinear grids

[Gyoshi]

surface currently accepts curvilinear grids. It would be nice if contour and contourf does too for plotting of parameterised values.

contours would also use VertexSurfaceLike a la #748
Similar to #675.

For example, the pyplot backend via Plots.jl does this:

import Plots; Plots.pyplot()
r1, r2 = 1.5, 1/2
r(u,v) = ((r1 + r2*cos(v))*cos(u), (r1 + r2*cos(v))*sin(u), r1/2*sin(u)+r2*sin(v)) # torus
us = vs = range(0, 2pi, length=25)
xs, ys, zs = [[pt[i] for pt in r.(us, vs')] for i in 1:3]
Plots.contourf(xs, ys, zs)