When used as a mask, Krita should recalculate additional
need/changeRect of the processed area of size:
actualSize + 4 * radius, which is too much of work.

In older version of Krita the test file loaded fine, because
colorize mask did update itself on loading and didn't provoke
full mask update. If you provoke full gaussian mask update
manually, you'll get the same 12-minutes recalculation process
(and it cannot run in threads, because access rects intersect
heavily).

The patch does the following:

1) When loading filter masks and filter layers: forcefully
   (and silently) limit gaussian blur size to 100 px.

2) When creating filter masks and filter layers, limit radius
   slider to 100 px.

3) When changing properties of Gaussian Blur mask, limit the
   radius slider to 100 px.

4) When applying Gaussian Blur filter directly, allow the user
   to select radius in full range: 0...1000 px

BUG:407062