Properly render gradients when strokeScaling = false

[adroitwhiz]

Description

In order to render gradients properly when strokeScaling is set to false on their corresponding items, their points must be transformed by strokeMatrix, which refers to the transformation matrix to be applied directly to the item's contents (path points, etc) as opposed to the canvas on which the item is drawn.

Related issues

• Relates to Gradients don't render properly on items where strokeScaling = false #1821

Checklist

• New tests added or existing tests modified to cover all changes
• Code conforms with the JSHint rules (npm run jshint passes)