ogl: Use a default program in programmable pipeline mode.

Each display has a default program for use when there to no
ALLEGRO_SHADER in effect.  Bitmaps act as they do in non-programmable
pipeline mode, by default, not requiring an ALLEGRO_SHADER to draw to
them.
ogl: glsl_use_shader forgot to look up variable locations after using
the default program.
ex_shader_target: Test previous bug fix.