canvas: Better coordinate rounding in scaling

When scaling in pixman you give the source coordinates in transformed
space rather than in the source coordinates. This is a bit problematic
when both source and destination coordinates are at integer positions, but
the scaling factor is not an exact 16.16 fixed point value. We used
to calculate the transformed source based on the floating point
transformation, which gave the wrong answer sometimes. Now we do the
calculations based on the fixed point transform that we give pixman.

However, even with this patch I can still sometimes see issues related
to this, although they are less bad.

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