This commit renames the GHOST Metal graphic context class and related
files / references from `GHOST_ContextCGL` to `GHOST_ContextMTL`. When
the Metal backend was first introduced, this context file contained
both the old OpenGL context (CGL, for macOS Core OpenGL API), and the
newer Metal context. Since #110185 all CGL related code was removed
from the class, making it Metal only, and thus rendering the old class
name outdated and potentially misleading. In addition to the rename,
unused OpenGL related forward declarations and old TODOs were also
removed.
Pull Request: https://projects.blender.org/blender/blender/pulls/142519
d95c2bc406