Change APIs for IDocumentRenderer

[clown]

A Page object may require read access to the dependent PDF library (PDFium, iText, etc), which may degrade performance. Therefore, we will change the API of the IDocumentRenderer interface as follows

6.0.0 (current version)

void Render(Graphics dest, Page page, PointF point, SizeF size);
Image Render(Page page, SizeF size);

7.0.0

void Render(Graphics dest, int pagenum); // may be extended method.
void Render(Graphics dest, int pagenum, Angle delta);

Image Render(int pagenum); // may be extended method.
Image Render(int pagenum, Angle delta); // may be extended method.
Image Render(int pagenum, double scale, Angle delta);
Image Render(int pagenum, SizeF box, Angle delta);

We will define methods of the current version as extended methods (Cube.Pdf.Mixin namespace).