win32: cleanup OpenGL context creation

Might handle some corner cases better. In particular, always call
wglMakeCurrent() with non-NULL HDC argument.
win32: create OpenGL context on the GUI thread

This fixes the fullscreen issues on Intel for me. I'm baffled by it and
don't understand why this suddenly works. Intel drivers being shit?
Windows being shit? HWND or HDC being only 97% thread-safe instead of
98%? Me missing something subtle that is not documented anywhere?

Who knows.

Now instead of creating the HDC and OpenGL context on the renderer
thread, they're created on the GUI thread. The renderer thread will
then only call wglMakeCurrent, SwapBuffers, and OpenGL standard
functions.

Probably fixes github issue #968.
build: move def file to libmpv/

This is more consistent with mpv.pc, which is also in libmpv/.