ImageBuffers aren't clipped against the visible frame size (RenderView).
This leads to unnecessary large allocations, which may crash some machines in extreme cases like <mask width="10000"....
As this worked at some point, before the ImageBuffer restructurization, this is actually a regression, so P1.
Created attachment 16722 [details]
Attached patch fixes crash - but not only for mask, but any ImageBuffer based content (ie. patterns, gradient on text, etc.)
*** Bug 15396 has been marked as a duplicate of this bug. ***
Comment on attachment 16722 [details]
Landed in r26745.
Regression per Comment #0.