Tiles optimisation

[Boudewijn Rempt]

I was reading through the autolayer doc and read:

As there is no way for us to know which pixel have true tiles and which have
the default, we would write to all the pixels within the extent, which would
automatically create ALL the tiles inside the extent.

Would it be a possible and useful optimization to add some logic in the tile 
manager (i.e., transparent to the rest of Krita) that does the following:

if write to a not-yet-created tile
	if pixel value == default pixel
		skip creation of tile & write

?

Of course, the two extra if's may well negate any advantage this scheme may 
have except for saving some memory.

By the way, when messing about with filters, I noticed that a simple filter 
action can easily double the width of a tile extent beyond the image size. 
I've added some checks to make filters only filter within the visible range 
of the image to prevent that, but that's no solution especially if other 
operations also make paint device extents grow without bounds. Maybe we can 
add some tightening here.
-- 
Boudewijn Rempt 
http://www.valdyas.org/fading/index.cgi
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