Bug: SurfacePattern is rasterized when using EXTEND_REPEAT on PDF.

Hi,

I have a bug in WeasyPrint (converts web documents to PDF): repeated SVG 
backgrounds are rasterized. (They appear as pixels instead of vectors.)

The source of the bug is in cairo or Pycairo. When a vector-type surface 
(PDFSurface or SVGSurface) is used as a source through a SurfacePattern 
and that pattern has EXTEND_REPEAT, the pattern is rasterized.

Attached is a test case that uses Pycairo to create an 'output.pdf' file 
in the current directory. When zoomed in a PDF viewer, the line on the 
left (not repeated) is in nice crips vectors, while the one on the right 
(with EXTEND_REPEAT) is in blurry pixels.

I had a quick look I’m not very familiar with cairo’s code base. In 
which file is EXTEND_REPEAT implemented? Would this be hard to solve?

Regards,
-- 
Simon Sapin

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import cairo

# Draw something on an abstract surface
s1 = cairo.PDFSurface(None, 100, 100)
c = cairo.Context(s1)
c.set_line_width(3)
c.move_to(10, 10)
c.line_to(20, 90)
c.stroke()

s2 = cairo.PDFSurface('output.pdf', 100, 100)
c = cairo.Context(s2)
# Non-repeated SurfacePattern
c.set_source_surface(s1, 10)
c.move_to(10, 10)
c.paint()

c.set_source_surface(s1, 20)
# Repeated SurfacePattern, rasterized
c.get_source().set_extend(cairo.EXTEND_REPEAT)
c.paint()