Main loop template for fast single pass bilinear scaling

Main loop template for fast single pass bilinear scaling

Can be used for implementing SIMD optimized fast path
functions which work with bilinear scaled source images.

Similar to the template for nearest scaling main loop, the
following types of mask are supported:
1. no mask
2. non-scaled a8 mask with SAMPLES_COVER_CLIP flag
3. solid mask

PAD repeat is fully supported. NONE repeat is partially
supported (right now only works if source image has alpha
channel or when alpha channel of the source image does not
have any effect on the compositing operation).