Reduce window rectangles cap to 8

Lowers the cap to 8 and adds a warning message if this value is ever
exceeded. The largest (only) implementation currently supports 8, so
there isn't yet reason to go higher.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2254013002

Review-Url: https://codereview.chromium.org/2254013002
diff --git a/src/gpu/GrCaps.cpp b/src/gpu/GrCaps.cpp
index 622c685..9f92210 100644
--- a/src/gpu/GrCaps.cpp
+++ b/src/gpu/GrCaps.cpp
@@ -141,6 +141,8 @@
         fMaxTileSize = options.fMaxTileSizeOverride;
     }
     if (fMaxWindowRectangles > GrWindowRectangles::kMaxWindows) {
+        SkDebugf("WARNING: capping window rectangles at %i. HW advertises support for %i.\n",
+                 GrWindowRectangles::kMaxWindows, fMaxWindowRectangles);
         fMaxWindowRectangles = GrWindowRectangles::kMaxWindows;
     }
 }
diff --git a/src/gpu/GrWindowRectangles.h b/src/gpu/GrWindowRectangles.h
index 682af93..f83f8dd 100644
--- a/src/gpu/GrWindowRectangles.h
+++ b/src/gpu/GrWindowRectangles.h
@@ -13,7 +13,7 @@
 
 class GrWindowRectangles {
 public:
-    constexpr static int kMaxWindows = 16;
+    constexpr static int kMaxWindows = 8;
 
     enum class Mode : bool {
         kExclusive,