Modernize

[dyollb]

This pull request modernizes the code a bit, replacing member initialization lists by default member initialization. This is related to https://github.com/syoyo/tinygltf/commit/7a570c88d996918489bbbd3af7ebbda1f3f74a0a#r119070408

I used clang-tidy to refactor the code. The following checks were used:

• modernize-use-default-member-init
• readability-redundant-string-init
• modernize-use-equals-default
• cppcoreguidelines-prefer-member-initializer

[syoyo]

Nice! Each changes seems trivial, but lots of changes... Need some time to review it and ensure no side-effect exists.

[syoyo]

Run some tests and seems no side-effects observed, so merged!