Shader discard doesn't seem to work well on mobile, remove it

* Instead we write transparent black when out of bounds. This only works
  when we're blending, but the cases that we don't blend are when we do
  a blit to a precisely sized output target with no offset, so it
  shouldn't cause a problem.

renderdoc/data/glsl/texdisplay.frag

@@ -92,14 +92,18 @@ void main(void)
 	{
 		// by convention display 1D textures as 100 high
 		if(scr2.x < 0.0f || scr2.x > 1.0f || scr2.y < 0.0f || scr2.y > 100.0f)
-		   discard;
+		{
+			color_out = vec4(0, 0, 0, 0);
+			return;
+		}
 	}
 	else
 	{
 		if(scr2.x < 0.0f || scr2.y < 0.0f ||
 		   scr2.x > 1.0f || scr2.y > 1.0f)
 		{
-			discard;
+			color_out = vec4(0, 0, 0, 0);
+			return;
 		}
 	}