title: Using GL_SRC1_COLOR in glBlendFunc()
body: <p>I am using glBindFragDataLocationIndexed() and writing two colors in fragment shader. Following is my code :</p>
  
  <pre><code>glBindFragDataLocationIndexed(shader_data.psId, 0, 0, "Frag_Out_Color0");
  glBindFragDataLocationIndexed(shader_data.psId, 0, 1, "Frag_Out_Color1");
  
  glActiveTexture ( GL_TEXTURE0 );
  LoadTexture(texture1);    
  glBindTexture ( GL_TEXTURE_2D, tex_id1);
  glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA,width, height, 0, GL_RGBA,GL_UNSIGNED_BYTE, texture1_data);
  
  glActiveTexture ( GL_TEXTURE1 );
  LoadTexture(texture2);
  glBindTexture ( GL_TEXTURE_2D, tex_id2);
  glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA,width, height, 0, GL_RGBA,GL_UNSIGNED_BYTE, texture2_data);
  
  
  glEnable(GL_BLEND);
  glBlendFunc(GL_SRC_COLOR, GL_SRC1_COLOR);
  
  glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
  </code></pre>
  
  <p>Fragment Shader is :</p>
  
  <pre><code>#version 150
  in vec2 texcoord;
  
  uniform sampler2D basetexture1;
  uniform sampler2D basetexture2;
  
  out vec4 Frag_Out_Color0;
  out vec4 Frag_Out_Color1;
  
  void main(void)
  {
      Frag_Out_Color0 = texture2D(basetexture1, texcoord);
  
      Frag_Out_Color1 = texture2D(basetexture2, texcoord);
  
  }
  </code></pre>
  
  <p>OGL3.3 spec says that :</p>
  
  <pre><code>Data written to the ﬁrst of these outputs becomes the ﬁrst source
  color input to the blender (corresponding to SRC_COLOR and SRC_ALPHA). Data
  written to the second of these outputs generates the second source color input to
  the blender (corresponding to SRC1_COLOR and SRC1_ALPHA).
  </code></pre>
  
  <p>I have passed GL_SRC_COLOR, GL_SRC1_COLOR as inputs to glBlendFunc(). I am not sure whether the result is correct or not. Please find attached images. Also if I pass GL_ONE,GL_ONE ,only first texture is rendered and no sign of second texture at all.
  How can I verify that rest of my code is fine (and blending is correctly done) ? </p>
  
  <p>Following are texture 1 , texture 2 and result respectively.</p>
  
  <p><img src="https://i.stack.imgur.com/pyI1P.png" alt="Texture 1"> <img src="https://i.stack.imgur.com/WyEix.png" alt="Texture 2"> <img src="https://i.stack.imgur.com/9SWy6.png" alt="Result"></p>