layout: fix build on i386

Due to the (lack of) precision of floating-point values, comparison results may differ slightly between architectures, leading to the check_stretching test failing when building for i386. This can be fixed by adjusting the value against which we compare the ratio between x/y scaling factors in calculate_transformation.

Changing the test condition instead would go against the point of setting a (soft) limit to 5%.

With this patch, squeekboard builds successfully on arm64, i386 and amd64.