Cairo arc alignment is a bit "bouncy"

[Seth Hillbrand]

The radius of the arcs drawn in cairo appear to depend on the angle of the arc (slightly). This creates the effect of "bouncing" arcs while drawing and mis-aligns the edges of the finished arc relative to OpenGL and gerber output.

Application: kicad
Version: (5.1.0-rc1-117-gdae7b03465), release build
Libraries:
    wxWidgets 3.0.4
Platform: Linux 4.19.0-2-amd64 x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
    Boost: 1.67.0
    OpenCASCADE Community Edition: 6.9.1
    Compiler: GCC 8.2.0 with C++ ABI 1013

Build settings:
    USE_WX_GRAPHICS_CONTEXT=OFF
    USE_WX_OVERLAY=ON
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_PYTHON3=OFF
    KICAD_SCRIPTING_WXPYTHON=ON
    KICAD_SCRIPTING_WXPYTHON_PHOENIX=OFF
    KICAD_SCRIPTING_ACTION_MENU=ON
    BUILD_GITHUB_PLUGIN=OFF
    KICAD_USE_OCE=ON
    KICAD_USE_OCC=OFF
    KICAD_SPICE=ON