styleitem: Base render node size on image
When using fractional scaling, we may end up with an image that is slightly different in size from the bounds of our item, since the image will always be in integer size but the item size can be fractional. This causes it to be rendered at a slightly different size from the texture size, which triggers interpolation and leads to glitches in the rendered result.
To avoid all that, rather than using our item's bounds for the node size, use the image size converted back to logical pixels as the node size. This may actually end up with us using a fractional node size, but ensures that when rendered we use the exact image dimensions.