Fix elg import breaking Windows. Slice primative produces sector or circle.

Bug fix: clear Texture.file_string as well as image if free_after_load is set True. Also import of egl that was dragging in xlib functions and stopped pi3d working on windows.

Improvement: Slice primitive in pi3d/shape which is like a slice of a pie-chart. If colours specified with rgb values greater than 1.0 they are treated as u8 0-255 values.