Implement difference clip rects with window rectangles

src/gpu/gl/GrGLCaps.cpp

Index: src/gpu/gl/GrGLCaps.cpp
diff --git a/src/gpu/gl/GrGLCaps.cpp b/src/gpu/gl/GrGLCaps.cpp
index 4d856ac9320d9b1b58e5a3e9f0d1d8b1eb4583d9..75ae77687975c595f879d01446b6bce8b8e7ac60 100644
--- a/src/gpu/gl/GrGLCaps.cpp
+++ b/src/gpu/gl/GrGLCaps.cpp
@@ -463,8 +463,6 @@ void GrGLCaps::init(const GrContextOptions& contextOptions,
 
     if (ctxInfo.hasExtension("GL_EXT_window_rectangles")) {
         GR_GL_GetIntegerv(gli, GR_GL_MAX_WINDOW_RECTANGLES, &fMaxWindowRectangles);

[Mark Kilgard, 2016/08/17 17:59:17]

I actually think protecting yourself from a large number of window rectangles is
a good idea.  We've had code get burned when something like the number of
texture units goes up a bunch (Skia is an example of this...).

On NVIDIA GPUs, 8 window rectangles are "free" (meaning as fast as not having
them when they do no discarding), but if an implementation supported many more,
it's likely to have some kind of cost to cycle more than a fixed number.

OpenVG has a minimum limit of like 32 or something.  It's unlikely that many
window rectangles can all be "free".

I recommend you cap at 8.  If an implementation has a higher limit, you'd be
well-served to evaluate the performance of that higher limit rather than just
accept it.  Chrome updates fast enough that moving to a higher limit on future
hardware will be fast.

-        // Protect ourselves against tracking huge amounts of window rectangle state.
-        fMaxWindowRectangles = SkTMin(31, fMaxWindowRectangles);
     }
 
     if (kPowerVR54x_GrGLRenderer == ctxInfo.renderer() ||