Implement difference clip rects with window rectangles

Plumbs the pipeline for window rectangles and uses them for a very
basic implementation of difference clip rects. This puts a common
Blink pattern on fast path, but we will still eventually need to make
more comprehensive use of window rectangles during clipping.

GTX 960 perf result:
                              gpu                     glinst4                 glinst16
desk_jsfiddlebigcar.skp       0.254 -> 0.177 [70%]    0.279 -> 0.197 [71%]    0.577 -> 0.196 [34%]
keymobi_sfgate_com_.skp       0.697 -> 0.513 [74%]    0.766 -> 0.451 [59%]    0.769 -> 0.597 [78%]
keymobi_blogger.skp           0.406 -> 0.314 [77%]    0.436 -> 0.292 [67%]    0.696 -> 0.319 [46%]
desk_pokemonwiki.skp          0.121 -> 0.098 [81%]     0.13 -> 0.105 [81%]    0.216 -> 0.097 [45%]
desk_wikipedia.skp            0.121 -> 0.098 [81%]     0.13 -> 0.104 [80%]    0.199 -> 0.104 [52%]
keymobi_androidpolice_co...   0.443 -> 0.382 [86%]    0.447 -> 0.398 [89%]    0.444 -> 0.396 [89%]
keymobi_booking_com_sear...   1 .15 ->  1.03 [90%]     1.17 ->  1.06 [91%]     1.17 ->  1.05 [90%]
keymobi_theverge_com.skp      0.417 -> 0.396 [95%]    0.426 -> 0.405 [95%]    0.429 ->   0.4 [93%]

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2251573002

Review-Url: https://codereview.chromium.org/2251573002

0 files changed, 0 insertions, 0 deletions