Fix DOMElement origin drift under camera zoom

[moufmouf]

This PR:

• Fixes a bug

Fixes #7329.

Describe the changes

A scene-level DOMElement with a non-zero origin (the default is 0.5, 0.5) drifted
away from its world position when camera.zoom !== 1, by originX * width * (1 - zoom)
horizontally and the equivalent vertically. The same DOMElement nested in a Container
did not drift — the two render paths were inconsistent.

Cause: in DOMElementCSSRenderer, camMatrix.translate(-dx, -dy) bakes the origin
offset into the CSS matrix in both branches. The parentMatrix branch correctly leaves
transform-origin at 0% 0% (offset applied once). The scene-level else branch also
set transform-origin: (100 * origin)%, applying the offset a second time. The two only
cancel when the camera-matrix scale is 1, so under camera zoom the residual
originX * width * (1 - zoom) shows up as drift.

Fix: make the scene-level path match the (correct) container path — keep
transform-origin at 0% 0% and bake the scale-aware offset into the matrix. The
scaleX / scaleY factor the parentMatrix branch already applied is correct for the
scene-level branch too, so it's hoisted up rather than duplicated:

var dx = src.width * src.originX * src.scaleX;
var dy = src.height * src.originY * src.scaleY;
// ...
if (parentMatrix)
{
    camMatrix.multiply(parentMatrix);
}

camMatrix.translate(-dx, -dy);

(tx / ty now stay '0%', so transform-origin is 0% 0% on both paths.)

The container-nested element is unchanged (no drift before or after), and scaled / rotated
scene-level elements now also stay correctly anchored at any zoom because the baked offset
is scale-aware.

A minimal repro is available here and can be copy/pasted:

https://phaser.io/sandbox/f0eb3f05

Disclaimer: this PR was mostly written by Claude Code but manually checked to be working (so AI assisted but hopefully not AI slop :) )