Perform glReadPixels with PBOs in the gpu, if PBOs are available.

gpu/GLES2/gl2extchromium.h

 // 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.
 
 // This file contains Chromium-specific GLES2 extensions declarations.
 
 #ifndef GPU_GLES2_GL2EXTCHROMIUM_H_
 #define GPU_GLES2_GL2EXTCHROMIUM_H_
 
 #include <GLES2/gl2.h>
@@ skipping 321 matching lines @@
 #endif
 #endif  /* GL_CHROMIUM_texture_compression_dxt5 */
 
 /* GL_CHROMIUM_async_pixel_transfers */
 #ifndef GL_CHROMIUM_async_pixel_transfers
 #define GL_CHROMIUM_async_pixel_transfers 1
 
 #ifndef GL_ASYNC_PIXEL_TRANSFERS_COMPLETED_CHROMIUM
 #define GL_ASYNC_PIXEL_TRANSFERS_COMPLETED_CHROMIUM 0x84F5
 #endif
+#ifndef GL_ASYNC_READ_PIXELS_COMPLETED_CHROMIUM
+#define GL_ASYNC_READ_PIXELS_COMPLETED_CHROMIUM 0x84F6
+#endif
 #endif  /* GL_CHROMIUM_async_pixel_transfers */
 
 /* GL_CHROMIUM_copy_texture */
 #ifndef GL_CHROMIUM_copy_texture
 #define GL_CHROMIUM_copy_texture 1
 
 #ifndef GL_UNPACK_COLORSPACE_CONVERSION_CHROMIUM
 #define GL_UNPACK_COLORSPACE_CONVERSION_CHROMIUM 0x9243
 #endif
 
@@ skipping 259 matching lines @@
 #endif
 typedef GLuint (GL_APIENTRYP PFNGLINSERTSYNCPOINTCHROMIUMPROC) ();
 typedef void (GL_APIENTRYP PFNGLWAITSYNCPOINTCHROMIUMPROC) (GLuint sync_point);
 #endif  /* GL_CHROMIUM_sync_point */
 
 #ifdef __cplusplus
 }
 #endif
 
 #endif  // GPU_GLES2_GL2EXTCHROMIUM_H_