Drawbot-skia

- Added `svg` export support for `saveImage()`
- Added `path.line(pt1, pt2)`
- Added `shadow()`, but it is not 100% compatible with DrawBot's in some unavoidable ways.

- Pinned optional pyffmpeg requirement to 1.6.1 for now.
- Added `linearGradient(...)`
- Added `radialGradient(...)` (limited to a single center point and a zero start radius)
- Add `random`, `randint`, `choice` and `shuffle` from the random module to the default namespace.

- Fixed a serious issue with the graphics state and multiple pages, that caused wrong paint properties on pages after the first.

- Added `frameDuration(duration)`
- Added `mp4` export support for `saveImage()`

- Added `path.removeOverlap()`
- Added `path.union(other)` and `path.__or__(other)`
- Added `path.intersection(other)` and `path.__and__(other)`
- Added `path.difference(other)` and `path.__mod__(other)`
- Added `path.xor(other)` and `path.__xor__(other)`

- Added `clipPath(path)`
- Added `path.pointInside(point)`
- Added `path.bounds()`
- Added `path.controlPointBounds()`
- Added `path.reverse()`
- Added `path.appendPath(other)`
- Added `path.copy()`
- Added `path.translate(x, y)`
- Added `path.scale(x, y=None, center=(0, 0))`
- Added `path.rotate(angle, center=(0, 0))`
- Added `path.skew(x, y=0, center=(0, 0))`
- Added `path.transform(transform, center=(0, 0))`
- Added `path.arc(center, radius, startAngle, endAngle, clockwise)`
- Added `path.arcTo(point1, point2, radius)`
- Added `path.drawToPen(pen)`
- Added `path.drawToPointPen(pen)`
- Added `path.text(txt, ...)`
- Fixed bug when `font()` was not set