LinearGradient
data class LinearGradient(val startX: Float, val startY: Float, val endX: Float, val endY: Float, val stops: List<GradientStop>) : PdfPaint
Linear gradient from (startX, startY) to (endX, endY), interpolating through stops in source order.
Coordinates are in the same space as the path commands they fill — i.e. the vector's viewport. The renderer scales both together when mapping into the destination rectangle.