st/wgl: Release stw_framebuffer::mutex after pbuffer creation.

st/wgl: Release stw_framebuffer::mutex after pbuffer creation.

This change releases the stw_framebuffer::mutex past creation of
the pbuffer stw_framebuffer. Without this change the pbuffers
lock is never released. Since on win32 mutexes are recursive, this
does not hurt as long as all actions on a context are done from
the same thread. But if, for example, context creation happens in
a different thread than usage, every access to the context will
block for ever.

Signed-off-by: José Fonseca <jfonseca@vmware.com>