Fix up horribly broken cairo scaling

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>