render/gles2: move color uniform from quad.vert to quad.frag

We have no use for a v_color varying. We can use the uniform
directly from the fragment shader without getting the vertex shader
involved.

render/gles2/shaders/quad.frag

@@ -1,7 +1,8 @@
 precision mediump float;
 varying vec4 v_color;
 varying vec2 v_texcoord;
+uniform vec4 color;
 
 void main() {
-	gl_FragColor = v_color;
+	gl_FragColor = color;
 }