GImage (FUISprite) issues incorrect draw call when imageFill is set to "non regular values".

[BrunoSilvaFreire]

Setting the fillAmount on a image as we do during a update was causing it to

During a very long debugging session, we found out that FairyGUI is issuing empty draw calls with empty indices. Note that the return statement was added by me as part of the debugging session, the result is the same.

Here is also the draw calls as presented by RenderDoc.

Here is the video of the flickering effect.

https://user-images.githubusercontent.com/12514666/121226473-98bafa80-c858-11eb-904d-265d228a1894.mp4

[xiaoguzhu]

will check it later