Translates global screen coordinate to coordinates relative to the panel.
See also Panel:LocalToScreen.
This function uses a cached value for the screen position of the panel, computed at the end of the last VGUI Think/Layout pass.
ie. inaccurate results may be returned if the panel or any of its ancestors have been repositioned outside of PANEL:Think or PANEL:PerformLayout within the last frame.
The x coordinate of the screen position to be translated.
The y coordinate of the screed position be to translated.