gtkgstsink.c: Drop workarounds needed for Windows

gtkgstsink.c: Drop workarounds needed for Windows

Since we are making GdkGLContext call the core wgl*() functions directly
instead of via libepoxy, drop the workarounds that we needed for notifying
libepoxy that wglMakeCurrent() outside of GDK/GTK was called.

This way, we clean up the code, and as a result, we can use the GstGL
APIs like the other platforms to query what GL api that is to be used.

For ensuring that things work between different threads, we now call
gdk_gl_context_clear_current() in place of calling wglMakeCurrent(xxx,
NULL), so that we make sure that there is no current GL context on a
thread outside of GstGL's thread, which Windows does not like.