Bug on spinning.html with webrender

[negge]

Using master servo with -w (webrender), the spinning.html drops some of the rectangles. This can be seen with the commands:

without webrender:
./mach run -r -- https://people.xiph.org/~negge/spinning.html
https://people.xiph.org/~negge/servo_spinning_nowr.png

with webrender:
./mach run -r -w -- https://people.xiph.org/~negge/spinning.html
https://people.xiph.org/~negge/servo_spinning_wr_bug.png

[larsbergstrom]

I saw this on some Android devices, too. Seemed to be tied to the border radius, as the problem went away on that device (Samsung Galaxy Tab 2) if I removed it:
https://github.com/larsbergstrom/webrender-demos/blob/use_raf/transparent_rects.html#L82

[cbrewster]

I set the rotation to a static -24deg and while resizing the window, it looks like the ones on the edge are being clipped prior to the rotation happening.

[cbrewster]

Adding P1 as I assume we want all the linked demos on the newtab page to work properly. Feel free to change it if it shouldn't be P1.

[jdm]

This doesn't feel like it impacts the demo in a meaningful way, so I'm dropping the "omg terrible user experience must fix" label :)

[cbrewster]

Better gif of the original issue:

[nox]

Closing as duplicate of #11970.