Implement EXT_draw_buffers WebGL extention support in command buffer.

gpu/command_buffer/service/framebuffer_manager.h

Index: gpu/command_buffer/service/framebuffer_manager.h
===================================================================
--- gpu/command_buffer/service/framebuffer_manager.h	(revision 186778)
+++ gpu/command_buffer/service/framebuffer_manager.h	(working copy)
@@ -114,6 +114,10 @@
   // Check all attachments are cleared
   bool IsCleared() const;
 
+  GLenum GetDrawBuffer(GLenum draw_buffer) const;
+
+  void SetDrawBuffers(GLsizei n, const GLenum* bufs);
+
   static void ClearFramebufferCompleteComboMap();
 
  private:
@@ -160,6 +164,12 @@
   typedef base::hash_map<std::string, bool> FramebufferComboCompleteMap;
   static FramebufferComboCompleteMap* framebuffer_combo_complete_map_;
 
+  // Hardwired to 16. Might be expanded in the future.
+  // This is not the max drawbuffers supported by the hardware, but the number
+  // of defined GL_DRAW_BUFFER*_ARB enum.
+  static const GLsizei kMaxDrawBuffers = 16;

[greggman, 2013/03/08 21:48:28]

We don't do this for textures, why do it for draw buffers?

In other words, texture units are limited to glGetInteger(GL_MAX_TEXTURE_UNITS)
not GL_TEXTURE31 which is way lower than many current GPUs.

[Zhenyao Mo, 2013/03/13 01:22:02]

Done.

+  GLenum draw_buffers_[kMaxDrawBuffers];
+
   DISALLOW_COPY_AND_ASSIGN(Framebuffer);
 };