When there is opacity ≠ 1 specified, the gradient is drawn and masked on a "Tiling Pattern". The tiling has BBox, XStep & YStep values equal to the dimensions of the page, to avoid any clipping or tiling repetition. Patterns are unaffected by the "Current Transformation Matrix" so the clipping rectangle is always aligned with the page borders (SMasks and Groups don't have this property). Problems could possibly happen if a gradient is embedded on multiple pages (this doesn't work anyway) of different size.
This pull request resolves the following issues:
- Fix use of opacity gradients with fillAndStroke (#214)
- Fix the reuse of the same gradient after transformations
- Fix clipping of some gradients with opacity
- Add 'transform' function to the gradient object
