Remove IOCompositor's current_pos field

[wusyong]

Describe the new feature:
I was tracing a bug that caused the cursor to constantly reset to the default when in animation. And I found current_pos isn't set after initialized, and it will always be (0, 0) pass to hit test.

While it can be fixed by setting the position properly, this is a good opportunity to remove it. I mentioned in previous issue that we want embedder that could control and handle more stuff by itself. Setting should be one of them. Unless the embedder explicitly moves the mouse or other event it prefers to, the compositor shouldn't perform a hit test.

Wdyt? Is it okay to remove it? Or we just set the position?