Add can-ignore-rect hint to clear call

src/gpu/gl/GrGLCaps.cpp

Index: src/gpu/gl/GrGLCaps.cpp
===================================================================
--- src/gpu/gl/GrGLCaps.cpp	(revision 12054)
+++ src/gpu/gl/GrGLCaps.cpp	(working copy)
@@ -48,6 +48,7 @@
     fIsCoreProfile = false;
     fFixedFunctionSupport = false;
     fDiscardFBSupport = false;
+    fFullClearIsFree = false;
 }
 
 GrGLCaps::GrGLCaps(const GrGLCaps& caps) : GrDrawTargetCaps() {
@@ -84,6 +85,7 @@
     fIsCoreProfile = caps.fIsCoreProfile;
     fFixedFunctionSupport = caps.fFixedFunctionSupport;
     fDiscardFBSupport = caps.fDiscardFBSupport;
+    fFullClearIsFree = caps.fFullClearIsFree;
 
     return *this;
 }
@@ -224,6 +226,10 @@
 
     fDiscardFBSupport = ctxInfo.hasExtension("GL_EXT_discard_framebuffer");
 
+    if (kARM_GrGLVendor == ctxInfo.vendor() || kImagination_GrGLVendor == ctxInfo.vendor()) {
+        fFullClearIsFree = true;
+    }
+
     if (kDesktop_GrGLBinding == binding) {
         fVertexArrayObjectSupport = version >= GR_GL_VER(3, 0) ||
                                     ctxInfo.hasExtension("GL_ARB_vertex_array_object");
@@ -648,4 +654,5 @@
     GrPrintf("Use non-VBO for dynamic data: %s\n",
              (fUseNonVBOVertexAndIndexDynamicData ? "YES" : "NO"));
     GrPrintf("Discard FrameBuffer support: %s\n", (fDiscardFBSupport ? "YES" : "NO"));
+    GrPrintf("Full screen clear is free: %s\n", (fFullClearIsFree ? "YES" : "NO"));
 }