egl: Improve driver selection.

egl: Improve driver selection.

The idea is to be able to match a driver using the following order

  try egl_gallium with hw renderer
  try egl_dri2
  try egl_gallium with sw renderer
  try egl_glx

given the module list

  egl_gallium
  egl_dri2
  egl_glx

For that, UseFallback initialization option is added.  The module list
is matched twice: with the option unset and with the option set.  In the
first pass, egl_gallium skips its sw renderer and egl_glx rejects to
initialize since UseFallback is not set.  In the second pass,
egl_gallium skips its hw renderer and egl_dri2 rejects to initialize
since UseFallback is set.  The process stops at the first driver that
initializes the display.