ghostscript: force interpolation when rendering (#855)

Specifying option --oversample tends to introduce upsampling in rendering
by rasterizing page to an higher DPI.

This upsampling improves OCR results, but a correct choice of interpolation
method can increase even more the OCR quality.

Ghostscript seems to use a nearest interpolation as default choice for pdf.
This method doesn't average new introduced pixels with original pixels
resulting in an almost similar image but with more pixels.

Providing -dInterpolateControl=-1 force switching interpolation on.

In this commit the above option is passed to all ghostscript rendering
calls.

After testing, rendering a page at same DPI with interpolation
enabled does not introduce significant time overhead.

time (repeat 40 gs -dQUIET -dSAFER -dBATCH -dNOPAUSE -sDEVICE=png16m \
	-dFirstPage=1 -dLastPage=1 -r100.000000x100.000000 \
	-dInterpolateControl=-1 -o /dev/null -dAutoRotatePages=/None -f pzII.pdf)
7,66s user 0,33s system 99% cpu 8,012 total

time (repeat 40 gs -dQUIET -dSAFER -dBATCH -dNOPAUSE -sDEVICE=png16m \
	-dFirstPage=1 -dLastPage=1 -r100.000000x100.000000 \
        -o /dev/null -dAutoRotatePages=/None -f pzII.pdf)
7,42s user 0,39s system 99% cpu 7,808 total

Ghostscript interpolation control reference:
https://www.ghostscript.com/doc/current/Use.htm

src/ocrmypdf/_exec/ghostscript.py

@@ -95,6 +95,7 @@ def rasterize_pdf(
             '-dSAFER',
             '-dBATCH',
             '-dNOPAUSE',
+            '-dInterpolateControl=-1',
             f'-sDEVICE={raster_device}',
             f'-dFirstPage={pageno}',
             f'-dLastPage={pageno}',