Make Texture.add() firstFrame check more explicit

Fixes issue phaserjs#4088 where a SpriteSheet created from a trimmed texture atlas frame returns the second frame when the first frame is requested.

Author: cirras

src/textures/Texture.js

@@ -154,7 +154,7 @@ var Texture = new Class({
         //  This is used to ensure we don't spam the display with entire
         //  atlases of sprite sheets, but instead just the first frame of them
         //  should the dev incorrectly specify the frame index
-        if (this.frameTotal === 1)
+        if (this.firstFrame === '__BASE')
         {
             this.firstFrame = name;
         }