stbt._match: Don't halve the size of very small template images
stbt._match: Don't halve the size of very small template images

This fixes some false negatives with small (~25px) template images.

Note that the source image must always be larger than the template
image, so we don't need to check that `image_pyramid` has as many levels
as `template_pyramid`.

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