This makes adding a new RenderingResource less pervasive. RenderingResources are mainly cached to control their lifetime between WebProcess and GPUProcess. Their concrete types are rarely used especially in WebProcess. So replace the pointers to NativeImage, DecomposedGlyphs and Gradient to just a pointer to RenderingResource. Add to these classes traits so RenderingResource can be casted to any of them.
<rdar://problem/108977457>
Pull request: https://github.com/WebKit/WebKit/pull/13530
Committed 263824@main (d67761f1060a): <https://commits.webkit.org/263824@main> Reviewed commits have been landed. Closing PR #13530 and removing active labels.