xlib: do not cache return value of glXChooseVisual/glXGetVisualFromFBConfig
The returned XVisualInfo from glXChooseVisual/glXGetVisualFromFBConfig
is being cached in XMesaVisual.vishandle (and unconditionally
overwritten on subsequent calls). However, these entry points are
specified to return XVisualInfo instances to be owned by the caller and
freed with XFree(), so the return values should not be retained.
With this change, XMesaVisual.vishandle is essentially unused and will
be removed in a subsequent change.
v2: update commit message
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>