build entire libskia; support SkStrokeRec, SkPath::arcTo and ConvertConics2Quads

[anthrotype]

Changes below include

• build and embed the whole libskia (static library for Linux/Mac, shared DLL for Windows) using skia's own build toolchain (gn and ninja); previously we were only building a subset of skia using setuptools, but it was too hard to maintain as skia keeps evolving. Note that gn tool requires python2 to be on the PATH (or use PYTHON2_EXE environment variable). If one does not want to build libskia from source when compiling the pathops extension module, one can export BUILD_SKIA_FROM_SOURCE=0. To specify a directory where a pre-built skia is located use SKIA_LIBRARY_DIR environment variable.

• 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)

• add an arcTo method to Path class, which models SVG elliptical arcs. Note that skia internally stores elliptical arcs using conic curves.

• add Path::convertConicsToQuads method to approximate conic curves using qudratic bezier curves, which can be drawn with fonttools pens.

[anthrotype]

Perhaps worth mentioning that building the whole of Skia library on Appveyor Windows CI now takes 25 minutes (multiplied by 6 for each python version and arch). I will probably end up building it once in a while and have the build script download the pre-built DLL for windows at least.