stbt._match_template: Lower threshold for downsampled images
stbt._match_template: Lower threshold for downsampled images

Downsampling the images for the pyramid templatematch can cause images
that would match at the original size to not match at the smaller size.

I have only seen this happen in the self-test test_press_visualisation,
which has images with very thin, very high contrast text. At the
original size, the template matched with first pass certainty of 1.0; at
half the size, the certainty was 0.799, just below the default threshold
of 0.8.

To avoid matching images that didn't match before, we still use the same
threshold when checking the images at the full size (i.e. the last step
of the pyramid algorithm); but at the intermediate steps dealing with
downsampled images, we lower the threshold by 0.20. This number is
purely based on the result from the test_press_visualisation self-test,
which is the only failure of its type I have observed with the new
pyramid algorithm, after checking many images in a private collection.

In theory this has a performance impact, because the region of interest
processed at subsequent levels in the pyramid algorithm will be larger.
But I couldn't measure any difference with tests/run-performance-test.sh
(perhaps because the particular source video & template image I was
using don't present many matches between the thresholds of 0.6 and 0.8).

LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,wmanley/stb-tester