Added missing interpolation types for GLTF animation channels

[benjitrosch]

I noticed that the GLTF animation loading only supported linear interpolation, so by adding support for cgltf_interpolation_type_step and cgltf_interpolation_type_cubic_spline this should cover most cases.

This was tested using robot.gltf by importing it into Blender, removing some keyframes, using constant and bezier interpolation, and then rerunning the models_loading_gltf.exe example. Step is easy to recognize, and I think the spline looks right as well, but I would love someone with discerning eyes to tell me if it looks accurate.

Here's videos of the exact same animation timeline with different interpolation applied:

• Step interpolation example:

2024-04-15.01-40-29.mp4

• Bezier interpolation example:

2024-04-15.01-42-00.mp4

• Linear example (to compare with the others):

2024-04-15.01-42-36.mp4

[raysan5]

@benjitrosch Nice improvement! Thank you very much!