(rt #124001) avoid flooring a second time in matrix transform interpolation
The linear interpolation done by matrix_transform_bg() (also called
when rotating images) called floor() on the ordinate both in the
caller and in the interpolation code, which seemed to confuse the compiler
supplied with the gcc supplied with 32-bit strawberry perl 5.26.0.
I suspect a value just barely below an integer was being stored in the
FPU and then rounded up to that integer when converted to a double,
which then caused an off-by-one error in interpolation.