Auto merge of #19651 - pyfisch:background-placement, r=emilio

Unify background placement code

Merges the implementations for background-image placement
from gradients and images. Add missing parts and fix bugs.

Now supported are the CSS properties:

* background-attachment (except for local value, see #19650)
* background-clip
* background-origin
* background-position-x/y
* background-repeat
* background-size

It should be noted that backgrounds are not clipped to
rounded border corners.
(This was done before but worked only in simple cases)
See: #19649

This solves the following issues:
closes #19626
closes #16657
closes #19482 (examples from http://lea.verou.me/css3patterns/ are rendered perfectly but the round border is completely ignored now)
closes #19577

- `./mach build -d` does not report any errors
- `./mach test-tidy` does not report any errors

I enabled a few tests with the first commit but I have written about a dozen manual tests I will try to turn into ref tests either before or after this patch lands.

@bors-servo try

The relationship between the different inputs is visualized in this flowchart:
![flowchart-background](https://user-images.githubusercontent.com/2781017/34394430-5a06c72c-eb59-11e7-9d51-3d23e2215f07.png)

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/19651)
<!-- Reviewable:end -->

components/gfx/display_list/mod.rs

@@ -729,6 +729,7 @@ pub struct GradientDisplayItem {
     ///
     /// Without tiles, the tile will be the same size as the background.
     pub tile: Size2D<Au>,
+    pub tile_spacing: Size2D<Au>,
 }
 
 /// Paints a radial gradient.
@@ -763,6 +764,7 @@ pub struct RadialGradientDisplayItem {
     ///
     /// Without tiles, the tile will be the same size as the background.
     pub tile: Size2D<Au>,
+    pub tile_spacing: Size2D<Au>,
 }
 
 /// A normal border, supporting CSS border styles.