Skia-pathops

Path.stroke no longer calls Path.convertConicsToQuads itself.
Clients can call that directly (passing a desired tolerance) if they don't want to deal with conics that may be introduced as result of the stroking operation.

- Added Path.transform to apply 3x3 matrix and return transformed Path (26).
- Fixed shadowed name in `pathops.operations` which caused 'TypeError: PathOp object is not callable' (25).

* We now build the whole libskia (static library for Linux/Mac, shared DLL for Windows) using skia's own build toolchain. The current libskia embedded is from the "chrome/m80" branch.

* Expose `SkStrokeRec` as `pathops.Path::stroke`, for transforming a stroked path into an outline; this supports all properties of SVG path's stroke attribute (width, cap, join, miter limit)

* Added `Path::arcTo` method, which models SVG elliptical arcs. Note that skia internally stores elliptical arcs using conic curves.

* Added `Path::convertConicsToQuads` method to approximate conic curves using qudratic bezier curves (e.g. so that they can be drawn with fonttools pens).

- Updated skia library to latest commit on master (google/skia3517aa7).
- Dropped support for Python 2.7
- Added support for Python 3.8

Updated PyPI credentials on Travis/Appveyor to fix uploading issue (14). No code changes.

- Fixed issue with double starting points due to float precision (13).
- Allow to `Path.dump(as_hex=True)` a Path to stdout using hexadecimal notation for floating point coordinates.
- Fixed issue with extra collinear points being kept after simplify operation (12).
- Updated skia library