Use glInvalidateFramebuffer() when it is supported.

src/gpu/gl/GrGLCaps.h

Index: src/gpu/gl/GrGLCaps.h
diff --git a/src/gpu/gl/GrGLCaps.h b/src/gpu/gl/GrGLCaps.h
index 21176d16dd80458278c1deb663885373d89af501..4cde2bd5f7317b8902a4a68e9ae6ac3f1991293e 100644
--- a/src/gpu/gl/GrGLCaps.h
+++ b/src/gpu/gl/GrGLCaps.h
@@ -75,7 +75,15 @@ public:
         /** GL_NV_shader_framebuffer_fetch */
         kNV_FBFetchType,
 
-        kLast_FBFetchType = kNV_FBFetchType,
+        kLast_FBFetchType = kNV_FBFetchType
+    };
+
+    enum InvalidateFBType {
+        kNone_InvalidateFBType,
+        kDiscard_InvalidateFBType,       //<! glDiscardFramebuffer()
+        kInvalidate_InvalidateFBType,     //<! glInvalidateFramebuffer()
+
+        kLast_InvalidateFBType = kInvalidate_InvalidateFBType
     };
 
     /**
@@ -159,6 +167,8 @@ public:
 
     FBFetchType fbFetchType() const { return fFBFetchType; }
 
+    InvalidateFBType invalidateFBType() const { return fInvalidateFBType; }
+
     /**
      * Returs a string containeng the caps info.
      */
@@ -307,9 +317,9 @@ private:
     int fMaxFragmentTextureUnits;
     int fMaxFixedFunctionTextureCoords;
 
-    MSFBOType fMSFBOType;
-
-    FBFetchType fFBFetchType;
+    MSFBOType           fMSFBOType;
+    FBFetchType         fFBFetchType;
+    InvalidateFBType    fInvalidateFBType;
 
     bool fRGBA8RenderbufferSupport : 1;
     bool fBGRAFormatSupport : 1;