clients/simple-egl: Allow translucent 16-bit surfaces

This allows users to get r5g5b5a1 configs which currently (in Mesa) interact
badly with EGL_EXT_present_opaque, and act as a good test case for the EGL
implementation.

Users can still get a 16-bit surface without alpha by using the "-o" flag.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>