This implements support for `glBlendEquation` and
`glBlendEquationSeparate`. These functions modify the calculation of the
resulting color in blending mode.
We now generate all LibGL API wrappers from a single API method
definition list stored in `GLAPI.json`. Since a significant portion of
the OpenGL API methods are relatively consistent variants, we take
advantage of this to generate a lot of these variants at once.
The autogenerated methods check for the non-nullness of the current
`GLContext`, and only perform an action if a `GLContext` is present.
This prevents a crash in ports like GLTron, who assume you can still
call the OpenGL API without an active context.
This increases our API wrapper method count from 211 to 356.
Fixes#15814.
