PDF export of clipped pattern adds raster image (large file size)

file is much larger than you would expect (1.4MB v 65KB for inkscape 0.91-1 on Windows 7, cmos_capacitor_91-1.pdf
reimporting back to Inkscape (internal import), we see that the file has large images, due to the 6 raster images that are used as masks for the pattern.
- extra issues (not the focus of this issue) are that the pattern is used 3 times, although in the original svg, there is just one; and the pattern is not visible on import back to inkscape.

no raster image is created to simulate the clipping, similar to previous behaviour on 0.91-1 on Windows 7

Original launchpad file (used above): cmos_capacitor.svg
- large export (1.4MB) cmos_capacitor_924.pdf
- small 0.91-1 export (64.5 KB) cmos_capacitor_91-1.pdf
Minimal file with issue cmos_capacitor_min4.svg
- large export (455KB) cmos_capacitor_min4.pdf
- small export when clip is removed from pattern's objects cmos_capacitor_min4_noclip.pdf
Note, minimal file from inkscape#86 (comment 367362691) also suffers the same issue

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