Merge gl::ObjectStore and gl::Config

[kkaefer]

These are currently two distinct objects, but they're almost always used together. Passing multiple objects around just makes the code longer and less readable. I'm not seeing any gains from keeping them separated.

[mention-bot]

@kkaefer, thanks for your PR! By analyzing this pull request, we identified @jfirebaugh, @tmpsantos and @ansis to be potential reviewers.

[kkaefer]

There are a few tasks that aren't necessarily part of this PR:

• move gl::debugging into gl::Context
• hide gl* calls in implementation files
• cleanup gl.hpp and gl.cpp and move various functions to their own headers

[jfirebaugh]

Looks great, gl::Context is exactly what #6468 needs to build upon.

CI failures look like minor include-what-you-use issues.

[jfirebaugh]

Oh, didn't look at Qt5 macOS failure, that's something more serious:

[ RUN      ] Annotations.UpdateSymbolAnnotationIcon
mbgl-test(11337,0x7fff7acf4000) malloc: *** error for object 0x7f9a1bcc8180: incorrect checksum for freed object - object was probably modified after being freed.
*** set a breakpoint in malloc_error_break to debug
make: *** [run-qt-test-*] Abort trap: 6

[kkaefer]

Yeah, I think we were destroying the Context before the AnnotationManager.