Build 'Convex' shape in memory from vertices

[Akshay5312]

Is your feature request related to a problem? Please describe.
There is no straightforward way to create a general 'Convex' shape given vertices.This means that to visualize shapes generated during runtime, one needs to generate a mesh file in memory (MemoryFile(formatted_string_containing_vertices, ".OBJ", filename_hint)) then parse that file into a Convex object. This is inconvenient.

Describe the solution you'd like
As suggested by @jwnimmer-tri, a Convex::Convex(const Matrix3d& vertices, const std::string& filename_hint, double scale = 1.0) constructor would be great. This would also help implement a method to convert VPolytopes to Convex shapes to register VPolytopes as geometries.

[jwnimmer-tri]

I imagine the best signature will be:

/** Creates a Convex shape from the convex hull of the given vertices,
 using an InMemoryMesh. */
explicit Convex(const Matrix3X<double>& vertices, std::string filename_hint = {}, const Vector3d& scale = Vector3d::Ones());

I think it's okay to default the hint to empty -- I don't think most users will need pass anything in since in most cases the Convex will be wrapped into a GeometryInstance with its own name.

For the scale we're moving towards non-isotropic scaling so I suggest Vector3d instead of double.

[Akshay5312]

For the scale we're moving towards non-isotropic scaling so I suggest Vector3d instead of double.

Currently, the Convex shape has a scalar 'scale' member (it is accessed and used otherwise as well). Changing this to a Vector3d would likely break a few things.

I could implement non-isotropic scaling by directly scaling the vertices before parsing them into a string?

[jwnimmer-tri]

Right. I imagined that for now, we would multiply the scale with vertices before making the wavefront string, and store scale_ = 1.0 internally. Then later once the member field is changed we could undo that. But maybe that's over-complicating things and we should just stick with scale double = 1.0 for now, and we can overload with a Vector3d constructor in the future.