Add discard API to SkCanvas, plumb it to glDiscardFramebuffer()

src/gpu/gl/GrGLDefines.h

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 
 #ifndef GrGLDefines_DEFINED
@@ skipping 673 matching lines @@
 #define GR_GL_FRAMEBUFFER_ATTACHMENT_GREEN_SIZE            0x8213
 #define GR_GL_FRAMEBUFFER_ATTACHMENT_BLUE_SIZE             0x8214
 #define GR_GL_FRAMEBUFFER_ATTACHMENT_ALPHA_SIZE            0x8215
 #define GR_GL_FRAMEBUFFER_ATTACHMENT_DEPTH_SIZE            0x8216
 #define GR_GL_FRAMEBUFFER_ATTACHMENT_STENCIL_SIZE          0x8217
 
 #define GR_GL_COLOR_ATTACHMENT0              0x8CE0
 #define GR_GL_DEPTH_ATTACHMENT               0x8D00
 #define GR_GL_STENCIL_ATTACHMENT             0x8D20
 
+// GL_EXT_discard_framebuffer
+#define GR_GL_COLOR                          0x1800
+#define GR_GL_DEPTH                          0x1801
+#define GR_GL_STENCIL                        0x1802
+
 #define GR_GL_NONE                           0
 
 #define GR_GL_FRAMEBUFFER_COMPLETE                      0x8CD5
 #define GR_GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT         0x8CD6
 #define GR_GL_FRAMEBUFFER_INCOMPLETE_MISSING_ATTACHMENT 0x8CD7
 #define GR_GL_FRAMEBUFFER_INCOMPLETE_DIMENSIONS         0x8CD9
 #define GR_GL_FRAMEBUFFER_UNSUPPORTED                   0x8CDD
 
 #define GR_GL_FRAMEBUFFER_BINDING            0x8CA6
 #define GR_GL_RENDERBUFFER_BINDING           0x8CA7
@@ skipping 173 matching lines @@
 #define GR_GL_ADJACENT_PAIRS                                0x90AE
 #define GR_GL_FIRST_TO_REST                                 0x90AF
 
 //path gen modes
 #define GR_GL_PATH_GEN_MODE                                 0x90B0
 #define GR_GL_PATH_GEN_COEFF                                0x90B1
 #define GR_GL_PATH_GEN_COLOR_FORMAT                         0x90B2
 #define GR_GL_PATH_GEN_COMPONENTS                           0x90B3
 
 #endif