[core] Use unique_resource for GL objects

[brunoabinader]

Source: https://github.com/okdshin/unique_resource

These replace the complexity of manually handling moveable-RAII objects with a type specific for that purpose.

As suggested in #5141 (comment).

👀 @jfirebaugh @kkaefer @tmpsantos @1ec5

[jfirebaugh]

Rather than using std::function polymorphism, create a unique *Deleter functor and Unique* typedef for each resource type as sketched in #5141 (comment). This adds additional type safety: mistaken use of the wrong resource type will be a compile time error.

[brunoabinader]

Changes since last review:

• s/gl::GLObjectStore/gl::ObjectStore/
• Using *Deleter functors and Unique* aliases for unique_resource usage.
• Dealt with absence of copy assignment in gl::ObjectStore reference inside all *Deleter's using std::reference_wrapper
• Dealt with absence of default constructor in unique_resource with mbgl::optional where needed (e.g. in places where the GL object is lazily created).

[jfirebaugh]

Shouldn't *buffer work, using the unique_resource implicit conversion?

[brunoabinader]

I kept .get() to keep in sync with the other Unique* usage elsewhere, but your suggestion looks better, thanks!

[tmpsantos]

@brunoabinader this is looking really nice. Do you think it would be possible to make utests for these holders? (not just that, but also the other wrappers we have for GL stuff)

[tmpsantos]

Neat, thanks for adding the tests.