Add support for surface pattern.

[hyowon]

I'm interested in implementing canvas features for servo.
SurfacePattern seems to be needed to support pattern fill for canvas.

[jdm]

This clone doesn't look safe; we'll have a dangling pointer if the clone outlives the original.

[pcwalton]

This will also be useful for making our tiled background painting code less hilariously slow.

[hyowon]

@jdm All patterns are cloneable now.

#[derive(Clone)]
pub enum Pattern {
Color(ColorPattern),
LinearGradient(LinearGradientPattern),
RadialGradient(RadialGradientPattern),
Surface(SurfacePattern),
}

I'm not sure whether we can remove clone from all patterns or not.
But CanvasPaintState having Patterns needs clone.

#[derive(Clone)]
struct CanvasPaintState<'a> {
draw_options: DrawOptions,
fill_style: Pattern,
stroke_style: Pattern,
stroke_opts: StrokeOptions<'a>,
/// The current 2D transform matrix.
transform: Matrix2D,
}

Is it difficult to keep on using clone while avoiding a dangling pointer?
I'm a newbie to rust and servo. Please advice me further. Thank you for your review.

@pcwalton Interesting. I'll also see the tiled background painting code.

[jdm]

Yeah, I just noticed that the other pattern types derive Clone as well, so they have the same problem. The way to fix it here is to add a C API to clone an AzSurfacePatternRef that casts it to gfx::SourcePattern and returns new SurfacePattern(sourcePattern->mSurface, sourcePattern->mExtendMode, sourcePatter->mFilter, sourcePattern->mMatrix, sourcePattern->mSamplingRect). We should do the same for the other pattern types, and manually implement the Clone trait instead of deriving it. Does that make sense?

[hyowon]

That makes great sense. Thank you for your kind guidance. Please review additional patch.

[jdm]

Looks great!

[hyowon]

cc @yichoi