This patch solves several problems introducing the possibility to
specify a row stride bigger than the width (visible pixels) on each
image row. Useful in case that we want to align the initial pixel
address of each row (if DOC_USE_ALIGNED_PIXELS is defined).
This allows us to use some SIMD intrinsics (e.g. SSE2) for some image
functions in the future (right now implemented only in the new
is_same_image_simd_templ() for is_same_image()).
Anyway to avoid breaking some existing code, by default we'll still
keep the old behavior: row stride bytes = width bytes (so
DOC_USE_ALIGNED_PIXELS is undefined).
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