Updated cubemap reflections example

[paweljarosz]

Removed unused assets, added more descriptions and explanations to the example, added screenshot and thumbnail, and modified the shaders a bit:

• The old shader used normal directly, which is only correct for an unrotated, uniformly scaled model.
• The old script pushed cameraPosition into the material every frame. That teaches an outdated pattern - Defold already gives the shader the view matrix, so the shader can derive camera position itself.
• The material is now explicit about local vertex space, matching modern model examples. The shader uses Defold’s per-instance mtx_world attribute to transform the model.
• Renamed envMap to env_map to match snake_case in other examples.

#191