gpu: Don't clear texture level in DoCopyTexSubImage2D() if not needed.

gpu/command_buffer/service/gles2_cmd_decoder.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "gpu/command_buffer/service/gles2_cmd_decoder.h"
 
 #include <stdio.h>
 
 #include <algorithm>
 #include <list>
@@ skipping 9127 matching lines @@
 
   ScopedResolvedFrameBufferBinder binder(this, false, true);
   gfx::Size size = GetBoundReadFrameBufferSize();
   GLint copyX = 0;
   GLint copyY = 0;
   GLint copyWidth = 0;
   GLint copyHeight = 0;
   Clip(x, width, size.width(), &copyX, &copyWidth);
   Clip(y, height, size.height(), &copyY, &copyHeight);
 
-  if (!texture_manager()->ClearTextureLevel(this, texture_ref, target, level)) {
-    LOCAL_SET_GL_ERROR(
-        GL_OUT_OF_MEMORY, "glCopyTexSubImage2D", "dimensions too big");
-    return;
+  if (xoffset != 0 || yoffset != 0 || width != size.width() ||
+      height != size.height()) {
+    if (!texture_manager()->ClearTextureLevel(this, texture_ref, target,
+                                              level)) {
+      LOCAL_SET_GL_ERROR(GL_OUT_OF_MEMORY, "glCopyTexSubImage2D",
+                         "dimensions too big");
+      return;
+    }
+  } else {
+    // Write all pixels in below.
+    texture_manager()->SetLevelCleared(texture_ref, target, level, true);
   }
 
   if (copyX != x ||
       copyY != y ||
       copyWidth != width ||
       copyHeight != height) {
     // some part was clipped so clear the sub rect.
     uint32 pixels_size = 0;
     if (!GLES2Util::ComputeImageDataSizes(
         width, height, 1, format, type, state_.unpack_alignment, &pixels_size,
@@ skipping 2481 matching lines @@
   }
 }
 
 // Include the auto-generated part of this file. We split this because it means
 // we can easily edit the non-auto generated parts right here in this file
 // instead of having to edit some template or the code generator.
 #include "gpu/command_buffer/service/gles2_cmd_decoder_autogen.h"
 
 }  // namespace gles2
 }  // namespace gpu