*bytes_per_pixel to provide proper alignment for each pixel
and row. This alignment is required to allow high-performance rendering
within cairo. The correct way to obtain a legal stride value is to
call <a class="link" href="cairo-Image-Surfaces.html#cairo-format-stride-for-width" title="cairo_format_stride_for_width ()"><code class="function">cairo_format_stride_for_width()</code></a> with the desired format and
maximum image width value, and then use the resulting stride value
to allocate the data and t