Sanitize DrawingCtx rect bounds
When created, RsvgDrawingCtx computes the transformed bounding box of the whole SVG and stores a cairo::Rectangle that holds it. This has float bounds.
However, in the rest of the code, we assume that
height are pixel sizes. We may thus be clipping off the last column/row of pixels here and there. Code which creates temporary surfaces should ensure that the bounds fit in the last pixel.