The differences between the mutter and GTK+ code are subtle, but it
turns out that _gtk_cairo_blur_compute_pixels actually returns the
shadow *spread*. Since we use a triple box blur, the constant was
multiplied by 1.5 to approximate three chained box blurs. Split this
out and use the correct value for the lobe width.
We weren't passing in the right "d" value, which was causing the blur to
behave incorrectly, especially in the case of 1px blurs, which would
cause no blurs at all.
The blur should now match the web.
Both Owen Taylor and I, the originators of this code, allow the code
herein be relicensed to the LGPL, which is what GTK+ ships with. This
prevents GTK+ from being GPL-only.
The code accesses pixels in a chunks of 4 bytes, so we must only support
formats where the size of a single pixel is 4 bytes.
Fix RGB24 to be 4 bytes (the alpha channel is ignored) and disallow A8.
