Remove transform attribute from images and paths

[LaurenzV]

Since all transforms on paths and images will be added in the form as a group, we won't really need them on Image and Path anymore. This makes the tree a bit simpler.

I hope I didn't miss anything.

[RazrFalcon]

I guess you can update crates/usvg/docs/spec.adoc as well. Remove it from image and note that only paths inside clipPath can have a transform.

[RazrFalcon]

If I remember correctly, transform is set only on groups because filter, mask and clip-path can be set only on groups. And

<g mask="..." transform="...">
   <path/>
</g>

is not the same as:

<g mask="...">
   <path transform="..."/>
</g>

The confusing part is that fill and stroke would still be set on the path, despite the fact they are also affected by the transform.

Groups in SVG is a very strange and counter-intuitive concept. I wrote about them a bit here and here.

[RazrFalcon]

Thanks!