When using XRENDER, create Xlib-based offscreen image surfaces.

Mozilla tried to enable XRENDER support with backends other than Cairo
in 286348:e13aaaaf1962 /
https://hg.mozilla.org/mozilla-central/rev/e13aaaaf1962 - at least until
they decided to completely remove XRENDER support.

The change looked innocent enough, but actually turned out to do exactly
the opposite: it forcefully enabled image offscreen surfaces with GTK2
when it was previously disabled (since
gfxPrefs::UseImageOffscreenSurfaces() will always return true) and, by
extension, disabled the XRENDER-based functionality by creating a
non-Xlib surface.

Interestingly, a previously enabled double buffering check was also
disabled by this, but since the comment for this was diverging with the
code, that behavior just sounds like yet another bug.

Instead of disabling image offscreen surfaces (at least when using the
GTK2 backend), let's force the creation of Xlib-based image surfaces
when XRENDER support is enabled. This will let UXP use the more
common/modern code paths, but also make scrolling much faster again. Too
fast scrolling may induce tearing (if not smoothed), but on the other
hand performs much better in remote computing contexts.

As an added benefit, GTK3-based builds should roughly behave the same
way. Further tests with the GTK3-backend enabled will be required in the
future.

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