Remove back-end source opengles2.cpp that is no longer used
Fix X11 mouse control, update README

Fix problem with X11 mouse panning that showed up with a recent xorg
server on an OpenGL-ES 2.0 platform by adding an XFlush call after
XWarpPointer the pointer.

Keyboard view panning (<, >) now works correctly.

Update README.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>