vo_opengl: Reword comment in shader

I didn't quite understand this comment after looking at the code again
months later, so I reworded it for better clarity.
gl_common: factor context creation

Always create the context in mpgl_init(), instead of doing it when
mpgl_config_window() is called the first time. This is a small step
towards cleaning up the GL backend interface, and adding other things
like perhaps GLES support, or a callback-driven backend for libmpv.
Do not call strerror()

...because everything is terrible.

strerror() is not documented as having to be thread-safe by POSIX and
C11. (Which is pretty much bullshit, because both mandate threads and
some form of thread-local storage - so there's no excuse why
implementation couldn't implement this in a thread-safe way. Especially
with C11 this is ridiculous, because there is no way to use threads and
convert error numbers to strings at the same time!)

Since we heavily use threads now, we should avoid unsafe functions like
strerror().

strerror_r() is in POSIX, but GNU/glibc deliberately fucks it up and
gives the function different semantics than the POSIX one. It's a bit of
work to convince this piece of shit to expose the POSIX standard
function, and not the messed up GNU one.

strerror_l() is also in POSIX, but only since the 2008 standard, and
thus is not widespread.

The solution is using avlibc (libavutil, by its official name), which
handles the unportable details for us, mostly. We avoid some pain.