Fix up horribly broken cairo scaling

The way cairo does scaling is really really inconvenient, and one of the
things in cairo that is fundamentally mis-designed.

Cairo scaling always affects both coordinates and object sizes, and the
two can apparently never be split apart.  Which is very much not what we
want: we want just coordinate scaling.

So we cannot use 'cairo_scale()' to scale our canvas, because that
screws up lines and text size too.  And no, you cannot "fix" that by
de-scaling the line size etc - because line size is one-dimensional, so
you can't undo the (different) scaling in X/Y.

Sad.  I realize that often you do want to scale object size with
coordinate transformation, but quite often you *don't* want to.

Yeah, we could do random context save/restore in odd places etc, but
that's just a sign of the bad design of cairo scaling.

Work around it by introducing our own graphics context with scaling,
which does it right.  I don't like this, but it seems to be better than
the alternatives.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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