title: Issue getting gradient square in glsl es 2.0, Gamemaker Studio 2.0
body: <p>I made a triangle list with 4 triangles, having the middle point a different color. And then aim to combine the triangles to get a nice gradient. 
  But the edges of the triangles create unwanted lines, I don't want these lines I want it to be smooth al the way.
  How can I get the desired result? </p>
  
  <p>Images:
  <a href="https://i.stack.imgur.com/R33Sh.png" rel="noreferrer"><img src="https://i.stack.imgur.com/R33Sh.png" alt="Unwanted lines"></a></p>
  
  <p>Shader Code: </p>
  
  <pre><code>    // Simple passthrough vertex shader
      //
      attribute vec3 in_Position;                  // (x,y,z)
      attribute vec4 in_Colour;                    // (r,g,b,a)
      attribute vec2 in_TextureCoord;              // (u,v)
  
      varying vec2 v_texcoord;
      varying vec4 v_colour;
  
      void main()
      {
          vec4 object_space_pos = vec4( in_Position.x, in_Position.y,         in_Position.z, 1.0);
          gl_Position = gm_Matrices[MATRIX_WORLD_VIEW_PROJECTION] * object_space_pos;
  
          v_colour = in_Colour;
          v_texcoord = in_TextureCoord;
      }
  
      //
      // Simple passthrough fragment shader
      //
      varying vec2 v_texcoord;
      varying vec4 v_colour;
  
      void main()
      {
          gl_FragColor = v_colour;
      }
  </code></pre>
  
  <p>Gamemaker code: 
  Create event: </p>
  
  <pre><code>    //Build vertices list
  
  
      vertex_format_begin();
      vertex_format_add_position();
      vertex_format_add_colour();
      vertex_format_add_textcoord();
      v_format = vertex_format_end();
      v_buff = vertex_create_buffer();
      vertex_begin(v_buff, v_format);
  
      //triangle 0
      vertex_position(v_buff, 200, 100);
      vertex_colour(v_buff, c_black, 1);
      vertex_texcoord(v_buff, 0.0, 0.0);
  
      vertex_position(v_buff, 600, 100);
      vertex_colour(v_buff, c_black, 1);
      vertex_texcoord(v_buff, 1.0, 0.0);
  
      vertex_position(v_buff,  400, 300);
      vertex_colour(v_buff, c_red, 1);
      vertex_texcoord(v_buff, 0.5, 0.5);
  
      //triangle 1
      vertex_position(v_buff, 200, 100);
      vertex_colour(v_buff, c_black, 1);
      vertex_texcoord(v_buff, 0.0, 0.0);
  
      vertex_position(v_buff, 200, 500);
      vertex_colour(v_buff, c_black, 1);
      vertex_texcoord(v_buff, 0.0, 1.0);
  
      vertex_position(v_buff,  400, 300);
      vertex_colour(v_buff, c_red, 1);
      vertex_texcoord(v_buff, 0.5, 0.5);
  
      //triangle 2
      vertex_position(v_buff, 600, 100);
      vertex_colour(v_buff, c_black, 1);
      vertex_texcoord(v_buff, 1.0, 0.0);
  
      vertex_position(v_buff, 600, 500);
      vertex_colour(v_buff, c_black, 1);
      vertex_texcoord(v_buff, 1.0, 1.0);
  
      vertex_position(v_buff,  400, 300);
      vertex_colour(v_buff, c_red, 1);
      vertex_texcoord(v_buff, 0.5, 0.5);
  
      //triangle 3
      vertex_position(v_buff, 200, 500);
      vertex_colour(v_buff, c_black, 1);
      vertex_texcoord(v_buff, 0.0, 1.0);
  
      vertex_position(v_buff, 600, 500);
      vertex_colour(v_buff, c_black, 1);
      vertex_texcoord(v_buff, 1.0, 1.0);
  
      vertex_position(v_buff,  400, 300);
      vertex_colour(v_buff, c_red, 1);
      vertex_texcoord(v_buff, 0.5, 0.5);
  
      vertex_end(v_buff);
      tex = sprite_get_texture(sprite_index, 0);
  </code></pre>
  
  <p>Draw event: </p>
  
  <pre><code>    shader_set(shd_prim);
      shader_set_uniform_f(uni_radius, var_radius);
      vertex_submit(v_buff, pr_trianglelist, tex);
      shader_reset();
  </code></pre>