Backport the GetImage crash fix to 1.20

This is a cautious version of what's in master. We do now always install miSourceValidate into the ScreenRec, but we still internally check that it is non-NULL before calling it, since existing 1.20 drivers will dynamically null that slot out, and expect it to work. Careful though: the second patch (with the actual fix) will call SourceValidate unconditionally, because none of the open drivers clear SourceValidate when initializing their screen, so we can still expect it to be set. (The closed drivers are expected to work the same way, though I've not checked.)