Fix the viewport fade effects with WebGL
Fix the viewport fade effects with WebGL

- The fragment shader now requires non-premultiplied alpha colors (like CanvasRenderer)

melonjs/melonJS,rlugojr/melonJS,srbala/melonJS,rlugojr/melonJS,rlugojr/melonJS,srbala/melonJS,srbala/melonJS,melonjs/melonJS