egl/main: Update README.txt
egl/main: Update README.txt

The driver search/load is not done at eglGetDisplay (or eglOpenDisplay
as the readme called it) time, but during eglInitialize().

Drop _eglMain (available only for external drivers) reference. Mention
we use function(s), specific to the built-in driver(s).

Signed-off-by: Emil Velikov <4c996f2f093ddd0585a556460b4cc3ba555546ef@gmail.com>
Reviewed-by: Marek Olšák <8c7344a1abdb103e79ecfd488098373070c3c70e@amd.com>
Reviewed-by: Chad Versace <a557f7f75bf28dc19ac99107f5cc7cd6b3fbdb1a@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler