feat(layers): let a layer slide animation start from a point of its own

[hm21]

Ports pro_video_editor#194 to the image editor's layer timeline, so a slide animation built here keeps matching the exported video.

What

A slide could only travel to and from a canvas edge, named by slideDirection. LayerAnimation.slideFrom now takes a start point instead:

LayerAnimation(
  type: LayerAnimationType.slide,
  phase: AnimationPhase.animateIn,
  duration: Duration(milliseconds: 600),
  slideFrom: Offset(-400, -400), // in diagonally from beyond the top-left
  curve: AnimationCurve.easeOutCubic,
)

The point may sit outside the canvas, and it overrides slideDirection when both are set — so nothing existing changes. animateOut leaves towards it, animateInOut does both.

How

The start point and the layer's resting place are both anchor points, so their difference is the distance travelled: the layer's own size cancels out, and with it the fractional component the edge-aware slide needs to clear the border. The preview composes a point slide as a plain pixel translation, which is what the native renderer does too.

The one deliberate difference from pro_video_editor: it measures its slideFrom from the video frame's top-left corner, because that is where its own layer offsets start. This one follows Layer.offset and measures from the canvas center. The doc comment says so.

Also

• slideFrom is serialized as {dx, dy} and carried through copyWith, equality and toString; fromMap stays lenient and ignores a non-map value.
• New safeParseOffset, the sibling of the existing safeParseSize, parses it.
• The slide assert now accepts either a direction or a point.

Testing

flutter analyze clean, dart format clean, all 594 tests pass — including 5 new preview cases (starts on the point, travels a linear fraction, settles at rest, overrides a direction, leaves back towards the point), 8 new model cases and a parser suite.