Add full support for KHR_animation_pointer validation

[aaronfranke]

Implements and closes #198.

This PR finishes the work that PR #230 started, providing full support for validating KHR_animation_pointer, including:

• Checking that the parent properties exist in the JSON.
• Checking that the component type and vector size match the expected ones.
• Support for all mutable glTF core pointers (the read-only ones are not included).
• Allowing extensions to integrate extension pointer validation.
• Integration for all extensions already in the validator, including the KHR_texture_transform interactions, which is everything in ObjectModel.adoc except for the EXT_lights_* extensions and the ADOBE_materials_* extensions.
• Test files for all of the above, including both positive and negative tests.

[lexaknyazev]

@aaronfranke I respect the efforts but certainly do not agree with the proposed implementation design. Going forward, please discuss non-trivial changes before proposing them.

[aaronfranke]

@lexaknyazev What is the alternative implementation design you propose?

[lexaknyazev]

The implementation proposed in this PR has multiple issues such as: hard-coded and duplicated logic, suboptimal use of the validator's API, several unreachable and untested sections, etc.

I have a quite different design drafted but not yet finished. May I borrow a few bits from this PR? This would speed up finishing that work.

[aaronfranke]

@lexaknyazev You may grab any code you want from this PR, or any future PR I make. Consider my work public domain.