Add support for gradient stops to wxGraphicsContext.

Add support for gradient stops to wxGraphicsContext.

Allow specifying a set of gradient stops instead of just the beginning and
ending colours. Add the new wxGraphicsGradientStop(s) classes and new
wxGraphicsContext::Create{Linear,Radial}GradientBrush() overloads. Also change
the same methods of wxGraphicsRenderer to take wxGraphicsGradientStops instead
of a pair of colours.

Implement the new API for MSW and Cairo. OS X still uses just the two colours
for now.

Closes #11897.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63857 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775