rendertransform.ts follows RenderTransform from accelerator.pyx: the same matrices in the same order, with
transform_anchor measuring the anchor in the child. Scene images are laid out from it (left/top plus a
matrix3d with origin 0 0), so rotation turns about the picture's centre and the box grows to its diagonal
as in Ren'Py; xpan/ypan are a wrapped shift of a repeating background, xtile/ytile repeat it, blend and a
fully additive picture become mix-blend-mode. Checked in a real browser page and by unit tests of the math.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
