[rs] Merge #594

594: Fix mipmap example which was y-flipped r=kvark a=Uriopass

Mipmap rendering was y-flipped, hence making one out of every two mipmaps wrong. See:

![image](https://user-images.githubusercontent.com/5420739/95760589-7eba1f80-0cab-11eb-8a5f-bfe9e228b480.png)

(look at the black dots)

Co-authored-by: Paris DOUADY <paris.douady@hotmail.fr>

wgpu/examples/mipmap/blit.vert

@@ -11,5 +11,6 @@ void main() {
         case 3: tc = vec2(0.0, 1.0); break;
     }
     v_TexCoord = tc;
-    gl_Position = vec4(tc * 2.0 - 1.0, 0.5, 1.0);
+    vec2 pos = tc * 2.0 - 1.0;
+    gl_Position = vec4(pos.x, -pos.y, 0.5, 1.0);
 }