[Galene] Background blur on Safari

[Juliusz Chroboczek <jch@irif.fr>]

Hi,

I've managed to get background blur to work on Safari.  I've tested on
Safari 18.2 for Mac OS X, I'll be grateful if people could tell me whether
it does the right thing on iOS.

The issue turned out to be Safari bug 198416:

  https://bugs.webkit.org/show_bug.cgi?id=198416

Safari silently ignores the "filter" property of Canvas2D, and therefore
doesn't perform blur when requested.  The result is that Galene would
painstakingly separate the foreground from the background, then fail to
blur the background, and finally recompose the two parts in order to get
what is essentially the original image.

It would probably have been a lot of fun to write a Gaussian blur as
a WebGL shader, I went for something significantly more low-tech: I scale
down the background by a factor of 36, then scale it back up.  While
upscaling, Safari applies what, to my untrained eyes, looks like
a bilinear filter, which yields a fairly acceptable blur.

The code checks for the availability of the "filter" property, not for the
browser version, so Safari users will get proper background blur as soon
as they get a version of Safari with the bug fixed.

Thanks again to Dirk-Willem van Gulik, who loaned me the Mac on which this
work was done.

-- Juliusz