Log Message:
mupdf: update to 1.21.1.

List of changes in MuPDF 1.21.1 (Source only release for PyMuPDF)

	Miscellaneous bug fixes, including:
		Garbage collection problem causing file bloat on clean.
		Don't assume sorted objects in pdf_objcmp.
		Don't layout empty documents.
		Type 3 font char bboxes.

List of changes in MuPDF 1.21.0

	Added MOBI input format support.
	Added Story API for creating PDF documents from formatted text.
	Added API to create, edit, and delete links.

	Support custom images for Stamp annotations.
	Support interior color on Polygon annotations.
	Support line endings on PolyLine annotations.

	Improved SVG output.

	C++/Python/C# binding:
		Changes to naming of wrapper functions and classes.
		Added limited support for callbacks into Python, using SWIG Directors.