Utilities to extract data from accessors

[forenoonwatch]

This PR adds fastgltf_tools.hpp, a header containing several utility functions such as getAccessorElement, iterateAccessor, and fillTargetFromAccessor, the ElementTraits<T> struct for matching metadata about the desired type, and the DefaultBufferDataAdapter, a minimal default implementation of the BufferDataAdapter pattern which allows the accessor indexing functions to resolve used buffer memory.

Currently this PR lacks support for compressed buffer views from EXT_meshopt_compression.

[spnda]

I have a few general things:

• Please adapt the else style to the rest of the repo, as well as the naming (currently snake_case) of the template specialisation constants.
• Probably should add some static_asserts that the required ElementTraits all exist, and that certain template types meet some criteria (could use concepts with a C++20 preprocessor guard).

[spnda]

• I'd like to see more comments around the top level functions and in some complex situations for the future.
• In Release mode on MSVC the "getAccessorElement" test fails sometimes (!) with sparse accessors. With a debugger I noticed the values are definitely wrong but I couldn't tell you why.

REQUIRE(checkValues[i] == fastgltf::getAccessorElementglm::vec3(*asset, secondAccessor, i))
with expansion:
{?} == {?}

[forenoonwatch]

• In Release mode on MSVC the "getAccessorElement" test fails sometimes (!) with sparse accessors. With a debugger I noticed the values are definitely wrong but I couldn't tell you why.

REQUIRE(checkValues[i] == fastgltf::getAccessorElementglm::vec3(*asset, secondAccessor, i))
with expansion:
{?} == {?}

I've been able to replicate this in my MinGW release (intermittently). Trying to see if I can actually figure out what's causing it.