The number of available clip planes depends on the OpenGL
implementation.
This change Introduces a new function in Qgs3DUtils function called
`Qgs3DUtils::getOpenGLMaxClipPlanes` to compute this
number (`GL_MAX_CLIP_PLANES`) and use it in the 3D scene.
See: https://registry.khronos.org/OpenGL-Refpages/gl2.1/xhtml/glClipPlane.xml
This adds two functions to setup clip planes on a 3d scene:
- enableClipping() to enable opengl clipping
- disableClipping() to disable opengl clipping
The `enableClipping()` function achieves 3 things:
- it stores the clipping plane equations
- it adds clip planes to the framegraph
- it enables the clipping on the material of the existing entities
`disableClipping()` does the opposite things.
The clip planes equations need to be stored to handle the dynamic
addition of new entities. Indeed, when a new entity is added, clipping
needs to be enabled on its material if necessary. This is achieved
in `finalizeNewEntity` which is called on all new entities.
However, this is not true for the terrain entity. That's why,
`mTerrain` now listens to the `newEntityCreated` signal to call
`handleClippingOnEntity()`.
