[svg] Cleanup text content model rules

Per spec (and empirically) <text> elements are not nestable (neither
directly or indirectly):
https://www.w3.org/TR/SVG11/intro.html#TermTextContentChildElement

Update the implementation to

 - only bridge onRender -> onRenderText in the root SkSVGText
   implementation
 - disallow <text> elements as text container descendants

Change-Id: I07b3abaf943b820e01c88f78bddf7ce5970ee508
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/358220
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Tyler Denniston <tdenniston@google.com>
2 files changed