Ellipsoids shape for volumetric primitives

[Speierers]

This PR adds support for two new shapes in Mitsuba that can be used to efficiently render volumetric primitives (e.g. 3D Gaussians): ellipsoids and ellipsoidsmesh. Those shapes defines a point cloud of anisotropic ellipsoid primitives
using specified centers, scales, and quaternions. The former employs a closed-form ray-intersection formula with backface culling while the later uses a mesh-based representation to leverage hardware acceleration ray-tracing.

This PR also provides volprim_rf_basic, an example integrator that renders the set of ellipsoids as a radiance field, similar to 3D Gaussian splatting. The more advanced integrators described in the Don't Splat Your Gaussian paper as well as example scripts will be open-sourced in a seperate repository.

Other contributions included in this PR:

• Add support for Shape::eval_attribute_X which returns a dynamic array of attributes
• Add the ParamsFlag::ReadOnly
• Add mitsuba.testing.RenderingRegressionTest for easily create regression tests similar to test_render.py
• Add the infrastruture to support per-shape backface-culling (currently only used for the ellipsoidsmesh shape.