WebGL 2: add read/write framebuffer binding points to related APIs

Source/platform/graphics/gpu/DrawingBuffer.h

Index: Source/platform/graphics/gpu/DrawingBuffer.h
diff --git a/Source/platform/graphics/gpu/DrawingBuffer.h b/Source/platform/graphics/gpu/DrawingBuffer.h
index 81c98223027a66456c277fb06c4bb9284ae22c54..2ce4693cc0744d891d92c99d3737a4ebdd4c9c2f 100644
--- a/Source/platform/graphics/gpu/DrawingBuffer.h
+++ b/Source/platform/graphics/gpu/DrawingBuffer.h
@@ -101,7 +101,7 @@ public:
 
     // Issues a glClear() on all framebuffers associated with this DrawingBuffer. The caller is responsible for
     // making the context current and setting the clear values and masks. Modifies the framebuffer binding.
-    void clearFramebuffers(GLbitfield clearMask);
+    void clearFramebuffers(GLbitfield clearMask, GLenum target = GL_FRAMEBUFFER);
 
     // Given the desired buffer size, provides the largest dimensions that will fit in the pixel budget.
     static IntSize adjustSize(const IntSize& desiredSize, const IntSize& curSize, int maxTextureSize);
@@ -110,7 +110,7 @@ public:
     IntSize size() const { return m_size; }
 
     // Copies the multisample color buffer to the normal color buffer and leaves m_fbo bound.
-    void commit();
+    void commit(GLenum target = GL_FRAMEBUFFER);
 
     // commit should copy the full multisample buffer, and not respect the
     // current scissor bounds. Track the state of the scissor test so that it
@@ -123,7 +123,13 @@ public:
 
     // The DrawingBuffer needs to track the currently bound framebuffer so it
     // restore the binding when needed.
-    void setFramebufferBinding(Platform3DObject fbo) { m_framebufferBinding = fbo; }
+    void setFramebufferBinding(Platform3DObject fbo, GLenum target = GL_FRAMEBUFFER)
+    {
+        if (target == GL_READ_FRAMEBUFFER)
+            m_readFramebufferBinding = fbo;
+        else
+            m_framebufferBinding = fbo;
+    }
 
     // Track the currently active texture unit. Texture unit 0 is used as host for a scratch
     // texture.
@@ -187,8 +193,9 @@ private:
     bool resizeMultisampleFramebuffer(const IntSize&);
     void resizeDepthStencil(const IntSize&);
 
-    // Bind to the m_framebufferBinding if it's not 0. Otherwise, bind to the default FBO.
-    void restoreFramebufferBinding();
+    // Bind to m_framebufferBinding or m_readFramebufferBinding if it's not 0.
+    // Otherwise, bind to the default FBO.
+    void restoreFramebufferBinding(GLenum target = GL_FRAMEBUFFER);
 
     void clearPlatformLayer();
 
@@ -226,6 +233,7 @@ private:
     bool m_scissorEnabled;
     Platform3DObject m_texture2DBinding;
     Platform3DObject m_framebufferBinding;
+    Platform3DObject m_readFramebufferBinding;
     GLenum m_activeTextureUnit;
 
     OwnPtr<WebGraphicsContext3D> m_context;