st/wgl: refactor framebuffer locking code

st/wgl: refactor framebuffer locking code

Split the old stw_framebuffer_reference() function into two new
functions: stw_framebuffer_reference_locked() which increments
the refcount and stw_framebuffer_release_locked() which decrements
the refcount and destroys the buffer when the count hits zero.

Original patch by Jose.  Modified by Brian (clean-ups, lock assertion
checks, etc).

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