Documented the methods of the wxD2DRenderTargetResourceHolder base class
Changed the way wxD2DContext::SetInterpolationQuality behaves

Instead of only working with Direct2D 1.1 this method now always saves the interpolation quality and defers the conversion to the method which does the actual drawing. This is because Direct2D 1.0 and 1.1 have different enums for representing interpolation modes.
Added preprocessor defines to ensure the functions declared in d2d1.h match the implementions in graphicsd2d.cpp
Renamed wxD2DConvertInterpolationQuality to wxD2DConvertInterpolationMode
Documented the need for wxD2DConvertInterpolationMode
Fixed warning regarding unreferenced formal parameters

Since composition modes are only supported in Direct2D 1.1, the implementation for 1.0 did not reference it's parameter.