Add ability to query read pixels support without a render target.

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 c8684d9d836b2dcdcb1e8addd72374eb5c66de2c..4467ef9a8bd45f5e682d57a746bda93ddb78a74c 100644
--- a/src/gpu/gl/GrGLCaps.cpp
+++ b/src/gpu/gl/GrGLCaps.cpp
@@ -642,11 +642,10 @@ bool GrGLCaps::hasPathRenderingSupport(const GrGLContextInfo& ctxInfo, const GrG
     return true;
 }
 
-bool GrGLCaps::readPixelsSupported(GrGLRenderTarget* target,
+bool GrGLCaps::readPixelsSupported(GrPixelConfig rtConfig,
                                    GrPixelConfig readConfig,
                                    std::function<void (GrGLenum, GrGLint*)> getIntegerv,
-                                   std::function<void (GrGLRenderTarget*)> bindRenderTarget) const {
-    GrPixelConfig rtConfig = target->config();
+                                   std::function<bool ()> bindRenderTarget) const {
     SkASSERT(this->isConfigRenderable(rtConfig, false));
 
     GrGLenum readFormat;
@@ -677,7 +676,9 @@ bool GrGLCaps::readPixelsSupported(GrGLRenderTarget* target,
         ReadPixelsFormat* rpFormat =
             const_cast<ReadPixelsFormat*>(&fConfigTable[rtConfig].fSecondReadPixelsFormat);
         GrGLint format = 0, type = 0;
-        bindRenderTarget(target);
+        if (!bindRenderTarget()) {
+            return false;
+        }
         getIntegerv(GR_GL_IMPLEMENTATION_COLOR_READ_FORMAT, &format);
         getIntegerv(GR_GL_IMPLEMENTATION_COLOR_READ_TYPE, &type);
         rpFormat->fFormat = format;