Nearest scaling fast path macro split into two parts

Nearest scaling fast path macro split into two parts

Scanline processing is now split into a separate function. This provides
an easy way of overriding it with a platform specific implementation,
which may use SIMD optimizations. Only basic C data types are used as
the arguments for this function, so it may be implemented entirely in
assembly or be generated by some JIT engine.

Also as a result of this split, the complexity of code is reduced a
bit and now it should be easier to introduce support for the currently
missing NONE, PAD and REFLECT repeat types.