On Mac and Win+DW, enhanced device contrast can possibly not be set.
This throws Skia for a loop by passing in an OOB scalar to its init.
Work around this by setting an accepted average value that helps
prevent font wash-out while not causing fringing.
Skia does some nasty things that break with DLL calls like having virtual dtors
without a body definition (and it's honestly surprising that it even builds to
begin with the way it's done, but apparently they use some linking symbol voodoo
to work around that in dependent classes).
Not wanting to have to pull Google code apart to try and fix that for something
we're only using for canvas anyway, it's better to just let Skia reside in
libxul instead.
In theory, a convex shape transformed by an affine matrix should still
be convex. However, due to numerical imprecision of floats, when we try
to determine if something is convex, we can get different answers pre
and post a transformation (think of two line segments nearly co-linear).
Convex paths take a faster scan converter, but it is only well behaved
if the path is, in fact, convex. Thus we have to be conservative about
which paths we mark as convex, and cant's trust transformed paths to
retain their convexity. We re-calculate when a transform is applied.
