Fix nested svg transform being applied twice

[StefanoD]

Fixes #1066.

Problem

A transform on a nested <svg> element was applied twice. The same happens to
other group properties (e.g. opacity). For example, a rect under
<svg transform="translate(50,50)"> was rendered at (100,100) instead of (50,50).

Cause

In convert_element, a nested <svg> was wrapped in a convert_group (which
reads the element's transform), and then convert_svg created its own group as
well — so the transform was applied in both. <use> does not have this problem
because it is routed to its handler and returns before the convert_group wrapper.

Fix

Route a nested <svg> directly to convert_svg and return early, exactly like
<use>. The outermost <svg> (no parent element) is unaffected.

Testing

• New regression test nested_svg_transform_applied_once (fails on main with
  translate(20,40), passes with the fix as translate(10,20)).
• The full usvg test suite and all 1724 resvg render tests pass.

Generated by Claude

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Note: I accidentally closed the original PR (#1067) by deleting my fork, which auto-closed it. This reopens the same change — the branch and commits are unchanged.