xlib: fix memory leak of and remove vishandle from XMesaVisualInfo

xlib: fix memory leak of and remove vishandle from XMesaVisualInfo

The vishandle member of XMesaVisualInfo is used to support the
comparison of XVisualInfo instances by pointer value, in
find_glx_visual().  The comparison however will always be false, as in
every case the comparison is made, the VisualInfo instance being
compared to is a new allocation passed in through a GLX API call.

In addition, the XVisualInfo instance pointed to by vishandle is itself
never freed, causing a memory leak.  Since vishandle is essentially
useless, we just remove it and thereby also fix the leak.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>