Add VDPAU_DRIVER_PATH support

Allow the user to specify the location of the backend driver,
via the VDPAU_DRIVER_PATH environment variable. This allows
easier testing of VDPAU backends without the need to rebuild
libvdpau.

Inspired by LIBGL_DRIVERS_PATH from mesa.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Aaron Plattner <aplattner@nvidia.com>
Signed-off-by: Aaron Plattner <aplattner@nvidia.com>