weston: Undo recent command-line change for renderer selection

weston: Undo recent command-line change for renderer selection

To restore command-line compatibility, reverting back to
integer-based renderer selection syntax:

weston [--use-gl=0|1] [--use-g2d=0|1]

Signed-off-by: Tom Hochstein <tom.hochstein@nxp.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>