Bad texture cache behaviour with large mask images

[jrmuizel]

Loading matklad/fall@931293e in Gecko with webrendest turned on causes about 1GB of A8 texture cache. This is presumably from large MaskLayers.

[nical]

This isn't really actionable on webrender's side other than general improvements to the texture cache tracked by #1486. I think that most of the work related to masks and texture cache needs to happen in gecko (by not generating mask images for rounded rectangles which seems to be the issue with that link).