We're using gcov+lcov, since these tools appear to be the only
usable ones that use the SC/CC metric; and measuring just the line
coverage would be practically criminal negligence.
gcov only works with GCC and Clang, and MSVC's own coverage
measurement tools are not up to the task; so MSVC is out of luck.
It was never really needed, since both Linux and OS X, where
GlOffscreen is used, guarantee that the API we need is present,
on all OS versions we're interested in.
Also, reorganize GlOffscreen consistently with the rest of our
codebase, and don't use RAII for OpenGL resource management because
of its requirement for an active context.
whitequark